Go LPG! .... Hints

Hints and tips for trouble-free LPG conversion and motoring

Here are some of differences and important points you need to be aware of if you plan to have your vehicle converted to run on LPG.

1. Condition of your vehicle prior to conversion

No matter which conversion company you choose, what type or which manufacturer's equipment you ask them to fit, it is best to begin with a properly serviced vehicle. Any problems that exist when running on petrol will still be there when running on LPG. An engine that is 'missing' or running inefficiently for whatever reason will do just that on LPG. This problem does not go away when conversion is complete. (For instance, some components e.g. spark plugs must be in better than average condition as LPG can be more demanding).

MOT Testing

We are often asked if the car should be MOT tested on LPG after conversion. The advice we offer is that it is always better to get the car tested on Petrol, ensuring that at least once a year it has been checked out for proper Petrol operation. If it does perform well on the test it should be capable of running properly on LPG, but if it doesn't it cannot run properly on either fuel and you will need to get it looked at.

2. Don't let your LPG tank run empty!

(Note that this rule only applies to 'single point' conversions, not multi point)

As tank contents reduce it follows that tank pressure will reduce. That means less fuel gets to your vapouriser and ultimately to the engine. If allowed to get to a critical level, the fuel/ air mixture can become so 'weak' (not enough fuel mixed in the air inducted) that the engine may 'backfire' through the induction system, especially when the throttle is opened wide from a low setting. This can cause problems ranging from a simple 'popping' sound right through to a large bang and damage to air filter boxes, airflow meters and anything else in the induction system. Most LPG conversion specialists will fit an anti - blowback damage protector in the form of a one-way valve to cover any malfunction but this can only protect your system from minor backfire occurrences. If you let your LPG tank run right down to empty your chances of this happening increase a hundredfold and it won't be the fault of the installer or the equipment that was fitted for you. You may normally expect fair warning of low LPG tank contents from your vehicle as the engine may well become 'sluggish', especially during acceleration (much the same as running out of Petrol). If this happens, don't wait for it to stop altogether. Switch over to Petrol rightaway!

(As LPG equipment continues to improve and designs progress it hoped that LPG systems will soon be designed to disengage automatically and default to Petrol when a pre-set pressure has been reached inside the tank, avoiding such problems altogether).Note that this problem is eradicated by the latest Romano LPG multi-point injection system.

3. LPG fuel gauges

At Go LPG! we have a wide experience of equipment from different manufacturers of different nationalities. As you'd expect, quality and design of many component parts varies greatly, but in one way they are consistent. LPG fuel gauges have to 'live' and work in a hostile environment whilst measuring and indicating tank contents of liquid gas at low temperatures and high pressures. (Petrol gauges have a relatively easy life! ) It has proven difficult to produce a completely accurate gauge design at the price most customers will pay. The result is a gauge mechanism that is generally inaccurate to say the least. You should not rely soley on gauge readings to judge fuel remaining and it is far better to zero your odometer trip meter at every filling of your tank. Aim to refill before your tank becomes empty to avoid the potential problems listed at (2). You'll soon develop a 'feel' for how much fuel your vehicle uses, especially as the interest in your new fuel cost saving grips you!

4. From cold, always run your engine up to full temperature on Petrol.

Some LPG installations will not give you this choice, but if you have one that will allow you to select LPG before the engine is up to temp. this can be asking for trouble, especially on damp days or during the winter months (or both). Be aware that on a cold morning the thermostat placed in the coolant circuit will be closed for much longer. It is unlikely that there will be any circulation of coolant around the heater circuit, and therefore no heat at all going into the vaporiser. Also remember that your vaporiser relies soley on heat from the engine's coolant system, if you have the heater going full chat to demist the screen as well as running on gas this will greatly increase the time it takes for the engine to get up to full temp.

Overall, it is much better to run up to temp. on Petrol, avoiding any icing problems associated with vapourising liquid gas and only switch over to LPG when the engine is fully up to temp. Stick to this and avoid a lot of potential problems! (This effect is more fully described in the item 'Will there be any difference in performance after conversion')

Hot water circulation stops the vaporiser from feeezing up If the vaporiser is not being heated it will be covered in a thick frost in seconds..... If you start up from cold on gas, the vaporiser is not being heated by engine coolant. There is a refrigerating effect as the gas vaporises. It will freeze up and stick, possibly causing weak mixture backfiring or gas leakage. If you run the engine up to temp. on petrol before switching over to gas this cannot occur as the engine coolant will already be hot. Don't be fooled into thinking that the surrounding air is too warm for freezing to occur, outside temperatures do not have to be down to freezing level for this to happen. The vaporiser may freeze at temperatures up to + 25 degrees Centigrade, especially if the air is very damp.

The picture of the frozen vaporiser only shows an extreme case. A vaporiser may be frozen internally whilst showing no sign of ice on its outer casing. Even if your vaporiser is only cold or luke warm to the touch it is in danger of freezing within seconds if you continue to run on LPG.

The hand test.....

If your engine splutters or stops whilst on LPG in cold or damp conditions, open the bonnet as soon as possible and feel the vaporiser's body with your hand. As a general rule, if it isn't so hot that you cannot comfortably hold a hand on it for more than a few seconds, it is not getting hot enough to work properly. If you cannot do anything about the lack of heat right there and then, switch to petrol and drive to your destination. You can sort out what has caused the lack of heat later. To investigate the cause, first read Section 7 thoroughly.

There are many more good reasons for cold starting only on Petrol, here are but a few -

Carburetted vehicles -

If you leave the carburettors dry for long periods it is likely that float valves (the things that shut off the petrol supply to the float bowl when it is full) will stick, resulting in petrol overflowing from the carburettor. Not good when you have to switch over to petrol as a reserve fuel at some point! Also, seals and gaskets may shrink if left dry for long periods. Cold petrol starting once a day will help to maintain these items and also stop varnish and gritty deposits building up in the float bowls.

Fuel Injected vehicles

Aside from the same effects of varnish build up and gritty deposit formation, gasket or seal shrinkage if left dry for long periods, the injectors themselves lose the valuable self-cleaning action that occurs when they are squirting fuel (a garden hose will not block with dust etc. whilst you are using it) when running on LPG. Start your engine once a day from cold on petrol, run it up to full temperature and you will clean the injectors, maintain the seals and prevent deposits building up in the fuel lines.

Is there a rule of thumb for suitable changeover temperature, time maybe?

Time on its own is of no use.

Let's say that 4 mins of petrol running produces 1KW of heat. In the Summer that may be more than enough if starting up from +20C because you would only have to maintain that temperature in the vaporiser.

In the winter and starting from - 5C it will not be good enough as you have to raise the temperature of the vaporiser from - 5C to +20C and you are unlikely to do that with 1KW. Another rule of thumb is required as time alone is demonstrated here to be of no practical use.

Waiting for hot air on the screen is a good plot, it shows there is hot coolant in the heater circuit and therefore the vaporiser.......Only switch when hot demist is available, summer, winter, anytime and you cannot go wrong.

Wouldn't the car's temperature gauge be a good thing to use?

In short, NO!

Temperature gauge information is only valid for the part of the engine its transducer is placed in - In the winter the thermostat could be completely closed with the temperature gauge reading +30C. This does NOT mean that the heater circuit is also at +30C, nor does it even mean that any coolant circulation is taking place in the heater circuit. It only means that coolant is up to that temperature around the transducer and that might be tucked deep in the engine galleries.

In this way the temperature gauge is of no real use when assessing changeover temperature.

Stick to the 'warm air on the screen' method and you cannot go wrong.

NOTE that all of the above advice is just as valid for EVERY type of LPG conversion, be it Open Loop, Closed Loop or Multi Point Injection. All of those systems have vaporisers and all of them will freeze if not heated sufficiently.

5. Keep your ignition components right up to scratch.

Items such as Distributor caps, HT Leads, Rotor arms, Coils etc. can appear to be perfectly serviceable but cause problems when running on LPG. This is because LPG / Air mixture is much harder to ignite than Petrol / Air mixtrure. The voltages generated in items such as HT leads are consequently up to twice as high ( one would expect to see spark voltages of 10-15Kv's on petrol, whereas on LPG they can reach 25 -30Kv's). Higher spark voltages are much more likely to break down the insulation of any component, causing 'tracking', misfiring etc. which often results in backfiring. Thus, faults that are NOT evident when running on Petrol can show up on LPG with quite serious effect.

If you have new components fitted (or fit them yourself) only use the manufacturer's original equipment. Spurious items may well be very good but there really is no substitute for the real thing. Using other parts can prove to be a false economy. Although all spark ignition engines can suffer misfiring caused by old or substandard ignition parts, in 6, 8, 10 and 12 cylinder ignition systems the spacing between the 'poles' on the distributor cap can be quite small. It is easier for a spark to 'jump' to the wrong pole as a result, (and through that, spark the wrong plug). It is crucial that this and other components do not cause this problem when running on LPG.

To give a fuller picture of the phenomenon, here is a brief explanation I gave on an LPG forum recently -

'Gas/air mix is much harder to ignite than a petrol air mix.

As a result, the voltages reached within the HT components (just before the spark occurs) can become far higher when running on gas as opposed on petrol, typically 12-15 Kv's on petrol and up to 30 - 35 Kv's on gas.

This means that the insulation of HT components is at least twice as likely to break down when running on gas, often resulting in tracking, leakage, misfiring etc.

This is the point where we often hear the statement 'It doesn't misfire/backfire/judder/stall on petrol'.

In the light of the above, it wouldn't.

To help reduce the potential for problems when running on gas, set plug gaps to .030" (.75mm).'

To show this effect in action you need to see what a reduction in plug gaps does on a 'scope.

Too wide = High voltage.

Reduce gap = Reduced voltage.

No one says that reducing the plug gap will dramatically remedy backfiring, but it will certainly reduce the risk.

Another factor often missed is that once HT leads etc. have been exposed to higher than normal voltages for some time they will have had their insulation damaged to the extent that a later reduction in plug gaps cannot stop the leakage. Misfiring and the ensuing backfire will still occur. 'Tracking' or 'Shorting' is, after all, mini lightning. You'd expect something to be damaged by this if only by the inherent erosion. Such damage makes just the place where damp likes to dwell, making things even worse.

If that is the case, only replacement of the damaged components will cure the problem (if the backfiring is really due to ignition faults in the first instance).

(Also see point 13 below)

NB! Go LPG only recommend the use of NGK spark plugs for all LPG operation, they are consistently the best plugs for gas applications. You can get expert advice on plug slection by ringing the NGK Technical Hotline on 01442 281000

5a. Ignition Problems Arising Just After Servicing....

This is so commonplace it is worth a mention. We have had many converted cars brought for attention after running just fine on LPG for 12-18 months. When an ignition problem is identified, the owner will often put two and two together and realise that the car began to misbehave shortly after it was serviced. The story then unfolds....Most often incorrect spark plugs have been fitted (not NGK or perhaps NGK but the wrong grade) and the gaps have not been reduced for LPG operation.

Lots of other components (e.g Rotor Arms, Coils to name but two) can cause problems only on LPG and Red Herring Number One crops up again and again - "It Runs OK on Petrol, so it must be the Gas System , Right?"


What may be OK for petrol operation may not be any use at all for LPG. I simply cannot count how many times I've seen this happen and if one thing is utterly accurate to say, it is this;
The Devil is in the Detail....

5b. 'Long Life' Spark Plugs

Maunfacturers of modern cars will try anything to gain advantage over others. The claim that spark plugs fitted to a late model will last for previously unheard of mileages (sometimes 50,000 miles) have a grain of truth in them. The electrodes are made of materials that withstand erosion (Platinum or Iridium are commonly used) to keep that all-important gap correct for a much longer period. That's great, but it is only a small part of the whole story.

The main reason for replacement of spark plugs is not erosion of the gap, but burning and subsequent corrosion of the plug body thread and the 'clump' of carbon formed at the tip of the thread. If plugs stay in an engine for too long, they may be impossible to remove due to the corrosion. Additionally, if they do begin to unscrew, the clump of carbon will damage the cylinder head theads as they are unscrewed. For these reasons I do not beleive the long - life spark plug and would rather fit cheaper, non Platinum or non-Iridium plugs, simply changing them more often. Then, at least we can be sure they will come back out....

6. Your engine must be capable of running on UNLEADED petrol.

This throws up an apparent anomaly, as it has been said that LPG has a much higher octane than raw petrol.

To find out why this must be the case, let's start by looking at what happens if you use unleaded petrol in an engine that hasn't been adapted. Over time, it will suffer 'valve seat recession', in other words the valve seats will become eroded, causing the valve to leak. To stop this the valve seats must be strengthened (the engine is often said to have an 'unleaded head' fitted.)

LPG will also cause recession on an unmodifed valve seat because the secondary function of lead in petrol was to cool and slightly lubricate the valve seats. Lead was not just added to petrol as an octane enhancer and the lack of it has similar consequences when running on LPG.

That said, recession will happen at a faster rate when running on LPG as the valve and seat temperatures are typically higher. The engine must be capable of handling unleaded petrol before conversion.

If head removal and seat strengthening are not an option, there is one alternative. You may have an upper cylinder lubricant injection system fitted. This will supply a special oil to the valves and seats when they most need it, but the system is then totally reliant on the driver making sure that the container does not run empty!

7. Make sure your coolant is always up to the correct level........

Vaporiser freezing season has begun.....!

In the last couple of weeks (writing on 19th Dec 2001) we have had no less than five requests to examine LPG systems that have either completely stopped the engine or made it run roughly after travelling just a mile or so into a journey. In one case, the driver lifted the bonnet after the engine stopped and observed that "The vaporiser was covered in white frost."

Wisely, he switched over to petrol and was able to drive to our premises. He complained - " The gas system has packed up".

With a little more examination, the vehicle's coolant level was found to be low. The vaporiser is normally quite high in the coolant circulation system and is often the first component to be starved of heat. As a result the vaporiser freezes up very quickly. (Outside temperatures do not have to be down to freezing level for this to happen, the vaporiser may freeze at temperatures up to + 25 degrees Centigrade, especially if the air is very damp).

Covered in a thick frost in seconds..... A vaporiser covered in a thick white coating of frost. It can freeze up within just a couple of minutes if its not heated by circulating engine coolant. This car hadn't covered a mile before the vapouriser failed and the engine stopped. The driver selected Petrol and was able to drive it to us quite normally. Note that when frozen, the vaporiser may even leak gas when the engine is stopped because gas metering and supply valves are frozen and stuck open.
Hot water circulation soon melts the frost As described, the vehicle's coolant level had become so low that the vaporiser was not being heated. Water was added to the cooling system and within a few minutes of running the engine on Petrol, the vaporiser thawed out. (Thawing is best done when running on Petrol because it prevents the vaporiser from further icing when doing its job of evaporating gas). When thoroughly thawed the vaporiser worked as normal and gas could be reselected.

Clearly there wasn't any fault with the gas system in this or any of the other cases. All that was lacking was a little routine maintenance................

Be sure to keep your coolant TOPPED UP to the correct level!

And there's more........!

OK, so the item above deals with the more common vaporiser freezing effects. For completeness there is another thing to mention -

A vehicle that has a full coolant system but less than 33% anti freeze protection is still at risk of vaporiser freezing, even when the coolant is circulating properly.

Why should that be?

Let's look at the facts.

An imaginary vehicle is running on LPG, travelling at motorway speeds in cold conditions, say 0 degrees centigrade (0 C). The windchill factor takes the temperature around the vaporiser down to - 5 C.

At this speed the vaporiser needs a lot of heat to do its job properly. If it does not get it freezing will result, as explained in the item above. The vaporiser will actually be a lot colder inside than the conditions outside, perhaps reducing internal temperatures by some 20 C.

The total temperature drop, including ambient temp (0 C ), windchill ( - 5 C ) and vaporiser refridgeration effect ( - 20 C) is an amazing figure - The inside of the vaporiser has the potential to drop to minus 25 C. If the car's coolant is not as hot as it would be in the Summer (and of course it will not be) the heat supplied may not be suffcient to overcome this huge temperature deficit and the vaporiser will actually begin to freeze the coolant circulating within it.

Let's assume that our hypothetical car has little or no anti freeze added to its coolant - What will happen then?

The coolant itself will begin to freeze, blocking the passages and galleries that allow its own circulation. As this happens, still less heat will be available to the vaporiser and it will freeze more of the coolant, further impeding circulation. This situation can only end one way if no other conditions change (outside temp., windchill factor, speed of the car (related to power demand and the resulting refridgeration effect of the vaporiser)).

The vaporiser will freeze enough coolant to completely block coolant circulation and the car will grind to a halt.....

As with the more normal type of vaporiser freezing, if the journey is continued on petrol the ice will eventually melt, concealing the evidence. The driver may think that the problem 'cleared itself', but in fact it looms to strike again the next time that set of conditions arise.

So, how do we avoid this problem?


Make sure that your car's coolant system contains at least 33% of a good quality antifreeze - 50% is better. You will not experience this problem with such protection!

7a. Vehicles with 'Closed' or 'Sealed' coolant systems

Most modern vehicles have a closed or sealed coolant system. We have recently heard of at least two such vehicles that developed 'airlock' long after conversion. (An airlock in a coolant system will prevent circulation in precisely the same way as it will in your household central heating radiators.) Their engines ran eractically on LPG and even stopped completely, due soley to the lack of hot coolant circulation and eventual vaporiser freezing.

What is the cause?

Time to examine the facts again.....

A closed coolant system that is properly filled with coolant and bled of any air at outset cannot allow the ingress of air later on as all of the space in the system is taken up by liquid coolant, save a tiny amount in the expansion tank.

The only way air can get into the system is if some coolant leaks out, possibly when the engine is hot. Air will later be 'sucked' or drawn in through the coolant leakage site after the engine has stopped and cooled down, an effect due to cooling, contraction and resulting low pressure in the system.

Both of the cars mentioned had coolant system leaks which allowed coolant to be lost with the engine hot and the coolant system was at full pressure (normally 5-7 PSI). This would certainly not show up as a simple leak when the engine was cold. When the engine later cooled down, air took the place of the lost coolant and predictably settled at the highest point in the system, which just happened to be inside the vaporiser.....The system was 'airlocked' as a result. This led directly to vaporiser freezing and the engine(s) ran badly or stopped altogether. A leak in the first car's system was found by having the coolant system checked with an air pump connected to the top of the expansion tank using a dummy cap. The system was pressurised, a leak on the bottom hose was quickly traced when coolant loss was observed.

The second car was not so simple.....It showed no leak during the pressure test, but continued to leak coolant whilst hot and later froze its vaporiser whilst being driven. The problem was eventually traced to a faulty seal on the expansion tank pressure cap, the only thing removed during the pressure test and therefore not checked.......!

The message here is plain.

When coolant loss on a modern car occurs, many will blame an LPG system if the engine runs badly or grinds to a halt.

The truth is quite the reverse......

The car has let down the LPG system by not supplying it with hot coolant.

Fix the leak, fix the problem.

Possible causes of coolant loss from a Closed or Sealed system -

Engine overheating
Hose leak
Water pump leak
Faulty expansion tank pressure cap
Leak in heater matrix or radiator
Crack in Expansion tank, Radiator, Engine block etc.
Cylinder induction and exhaust of coolant through cracked cylinder liner or other internal cracking.

NOTE that all of the above advice is just as valid for EVERY type of LPG conversion, be it Open Loop, Closed Loop or Multi Point Injection. All of those systems have vaporisers and all of them will freeze if not heated sufficiently.

8. Care for your petrol injectors whilst you are not using them........

A few cases of partially blocked petrol injectors in dual - fuel vehicles have encouraged us to investigate further.

The problem seems to occur mostly with larger engined cars that have covered a lot of miles before LPG conversion. Later, after running the car on LPG almost exclusively (as you would!) owners have found that petrol operation has become erratic and in one case the vehicle was practically undrivable on petrol, although it ran perfectly on LPG. Much examination showed that the injectors were blocked to the extent that their spray pattern was seriously affected. A miraculous improvement was afforded by the addition of a good quality injector cleaner to the petrol, followed by a few miles of petrol driving.

It appears that a car that has done a lot of miles (and maybe not had its injectors cleaned as often as it might) will have a 'skin' of carbon deposited on the injector heads. Whilst making no immediate difference when running on petrol (of course, all injectors will eventually become blocked if they are not cleaned) this coating can expand and harden when running the engine on LPG. Leaving your petrol injectors dirty and redundant for long periods is likely to cause problems later.

The problem is by no means sure to occur in any particular type of injector, vehicle or engine and the incidence is quite low (in the order of 1% of converted vehicles) but prevention is always better than cure.

OK, we are lucky enough to have our vehicles converted to run on a much less expensive fuel, but we still rely on petrol as a back - up. We want it to work when we do have to use it. To avoid problems altogether, there is a simple and inexpensive measure you can take right now!

Add an injector cleaner to your next tank of petrol (STP, Wynn's and other good brands are recommended) and your injectors will be cleaned up every time petrol is selected. If you are in the good habit of starting up on petrol and running the engine up to temp. before switching over to LPG, you will be preventing this problem from occurring. If you have an LPG system that will not switch over until a pre-determined temp. has been reached, you won't even have to give it a thought!

Someone suggested that the addition of injector cleaner to petrol is a 'hidden' cost of LPG use, one ignored by the conversion industry........

But injector cleaner is designed for use when running on petrol, LPG conversion or not!

8a. Always keep a 1/4 of a tank of Petrol in reserve.

This will not only benefit you if you run out of LPG, it has far more serious benefits.....

On most conversions the Petrol pump is still running whilst using LPG (for good reasons not discussed here). If the petrol pump runs dry whilst you are running the engine on LPG it can be burned right out. This is because the pump relies on Petrol to lubricate its bearings and keep it cool. Fuel 'slosh' when going round corners may leave a Petrol pump to run dry if there is only a gallon or so in your tank.

The 'Fail Safe' Mechanism

The most important factor to understand here is that if an unconverted car runs out of petrol the engine will stop. The car's management system will 'Fail Safe', turning off the petrol pump. This is an important protection device.

If the car is converted and running on LPG things are quite different. The 'Fail Safe' mechanism has been defeated. The petrol pump will continue to run even if there is no petrol in the tank simply because the engine is still running on another fuel. This is why we recommend that all converted cars have 1/4 of a tank of petrol at all times, just to keep the petrol pump 'wet' and lubricated.

Finally, make sure you use and replenish that 1/4 of a tank of Petrol in rotation - Petrol over 3 months old will have degraded to a greater or lesser degree and may not even be capable of running the engine at all! NOT what you want from a reserve fuel!

9. Beware of possible tank venting in the summer months.

Your LPG tank has (should have) a safety valve, designed to release the contents of the tank if the pressure within it rises for any reason. The prime objective of this valve is to allow the fuel to escape in a controlled manner should the tank be heated by a fire in the vehicle, thus avoiding a large explosion.

A typical Safety Valve, part of the Multi Valve unit fitted to the LPG tank.
The safety valve can also release some gas if a certain set of circumstances arise. This account of a recent ocurrence will help to illuminate the cause, prevention and cure -

In mid June, one of our customers recently filled his LPG tank at around 6 AM before going to work. His vehicle (Range Rover Classic) has its tank mounted in the rear load space.

As the day wore on the weather become quite hot and the sun was beating down on the black tank (the parcel shelf was not in place).

Black is the colour that will derive the most heat from any light that falls upon it. At around midday the tank began to vent LPG, doing so for several minutes.

Why did this happen and what could the owner have done to avoid the situation?

The answers are quite simple.

First of all, the tank was filled when the ambient temperature was quite low, in the early morning. This was fine until the Sun heated the tank. The contents expanded (became less dense) and the tank pressure exceeded the pre-set limit of the safety valve which reacted by venting some gas. When the pressure within the tank reduced below the pre-set valve's maximum level it ceased venting.

OK, so this could be at least very worrying for an owner or others, and is potentially dangerous, so what could have been done to stop this happening?

Well, first of all, if the tank had not been completely filled this may never have happened as more expansion space would have been available. Secondly, if it had been filled during the hottest part of the day, it also may not have happened because the pump would have stopped at the maximum permissible pressure under the higher temperature conditions.

Given that the tank was filled at the time it was, could the owner still have avoided the situation?


Avoidance / preventative measures

He could have parked the car in such a way that the Sun did not heat the tank, it is then unlikely that any venting would have occurred. If he could not do this, he could have covered the tank, insulating it from the Sun's heat. The lighter the colour of the cover, the more efficient it would have been. Silver reflective material is best at reflecting light (and therefore reducing solar heating of the tank). One of those reflective windscreen blinds would be perfect for this job.

Dealing with the occurrence (venting).

If the tank is venting, the first thing to do is to reduce the effect of the heat source, the Sun in this case.

A cold wet towel, sheet or blanket could be put over the tank, or if possible, cold water run directly over it. As the LPG content cools down it will shrink and the pressure within the tank will reduce, stopping the venting. At this time it (when venting has ceased) it would be wise to move the vehicle to shady spot and maybe even leave the engine running to further reduce the amount of fuel in the tank.

Notes -

Don't worry that the whole tank contents will be expelled if venting occurs - Once the pressure within the tank has reduced sufficiently the safety valve will close again.

Don't be put off using LPG as a fuel just because venting could occur one day - If you heed the advice given above the it'll never happen, it is quite a rare occurrence in any event.

Don't be misled into thinking that venting is an additional risk over petrol either - Petrol tanks can and do vent but this often remains unseen as the vapour expelled is largely transparent and not at any significant pressure.

Don't ever remove, or let anyone else remove or tamper with the lid of the airtight box around your LPG tank's multi valve (includes helpful souls, meddlers and the kids!). If the lid is left off the airbox any vented gas could be released inside the vehicle, the risks of which are clear.

10. Problems with filler adapters.

1. Fitting your adapter - If you have one of the types of LPG filler that need an adapter fitted before you can fit the UK filler hose, be sure that it is reasonably tight before trying to put any gas into the tank.

Several times we have heard of the filler leaking at the adapter joint face. The cause is either a worn sealing washer (normally black rubber type compound) which is cured by replacement of the washer or simply that the adapter was not screwed in tightly enough in the first instance.

If the adapter is not screwed in tightly enough to make a seal on both faces of the washer, gas will escape, freezing the washer. If this happens, the washer is practically useless as it becomes very hard when frozen, losing its ability to seal the joint faces. More leaks are inevitable.

There is absolutely no point in then trying to tighten the adapter further to make it seal - If the washer is frozen stiff it will not work, however much you tighten it. (Incidentally, this is what causes most of the wear on sealing washers.)

The only thing to do is to remove the adapter and warm it up somehow, thawing the washer and returning its flexibility. Run it under hot tap water, put it on a warm area of the engine (not whilst it is running!) or place it in the sun.......Use your imagination, but do not wring the life out of a frozen washer!

Having thawed out the washer, you can again try to fill the tank, maybe giving it a bit more of a tweak than you did before.

2. Watch out for twisted pump / filler hoses

They tend to straighten themselves when you let go to press the fill button, perhaps unscrewing the adapter and causing a filling leak. On the other hand, we have heard of filler adapters being overtightened by an unwinding pump / filler hose, making it impossible for the driver to remove the adapter after filling was completed. If this happens, don't use tools and force on the adapter in an attempt to free it as damage may result. Use the filler gun itself to loosen the adapter and then remove it as normal.

As always, prevention is a lot easier than cure. The best way to avoid either of the above problems is to straighten the pump / filler hose before you use it, making sure it is not twisted or kinked.

3. Never attach a wet filler gun....

If the filler gun is wet when fitted, ice will form when liquid gas lowers the temperature of the filler gun, and, more importantly, the water. As a result, the filler gun may be firmly frozen on to the filler point.

Always check the end of a filler gun before use and wipe it dry if it is wet.

11. Backfiring on fuel changeover (Converted vehicles with petrol injection).

It's a rare occurrence, but worthy of mention.

The changeover from petrol to gas is designed to happen only upon deceleration for very good reasons.

As the petrol supply is cut off, the engine is in the 'overrun' state, pressure in the manifold is very low. The last of the petrol is quickly inducted. The engine has NO fuel supply before the gas comes 'online'. It may take some time for the gas system to fully supply gas. (The vapour pipe will be full of air initially.) This can lead to a very weak initial fuel mixture.

If the driver has the fuel choice switch set to 'Gas' and decelerates, then increases throttle input again very soon afterward, the petrol supply has been cut, but the gas system isn't in a position to satisfy the engine's rapid demand for a vastly increased supply of fuel. (It is only supplying enough fuel for idle, and only that if it has already purged the air from the pipe). Indeed, for a short time, the engine is receiving NO fuel at all....

The result of this can be a weak mixture backfire.

The way to avoid this situation altogether is to either -

'Rev' the changeover to gas whilst stationary, allowing the gas system to stabilise its supply before a large demand for fuel is made, or -

Only select the 'Gas' position on the fuel choice switch when the engine is up to full operating temp. AND you have an opportunity to decelerate and then 'coast' for a few seconds, without having to make a sudden demand for a lot of fuel.

If you use one (or a mixture of both) of the above changeover procedures you will never meet the changeover backfire.

12. LPG Starting Procedure

(Carburetted vehicles only)

If you intend to start your engine on LPG (not recommended for many reasons, see point 4) remember that you should wait a few seconds for the vaporiser to 'prime' the mixer with gas prior to cranking.

Whilst you may start immediately on petrol, that is because some fuel is already in the carburettor (if you stopped the engine whilst running on petrol). Starting on LPG is a different matter altogether, as even if the engine were stopped whilst running on LPG, no gas can remain in the system.

The manufacturer of the LPG control system (often combined with the fuel choice switch) allows for this by giving the switch the ability to 'prime' the engine with a short burst of gas as soon as the ignition is switched on (normally a timer controlled burst of 5 seconds or so).

If you switch on the ignition and crank immediately, you have not allowed the priming to take place and a longer cranking time will be the result. Eventually, of course, the priming will be done as you crank and the engine will start, but it takes a good deal longer and can only accelerate the wear on your starter motor and ring gear.

To confirm your switch has this timed priming feature (maybe because you simply were not told about it when you had your conversion fitted) run this simple test -

Switch on your ignition whilst the switch is set for LPG starting. Listen for the opening of the solenoid(s) and a short hiss from the vaporiser. Wait until the hiss stops (within 5 seconds normally) and the solenoids close again. Now turn the key for cranking (do not turn the ignition off, as you will have to begin the whole procedure again). Your engine should start far quicker than if you had simply turned the key for immediate cranking.

It is hard to remember to do this and I have to admit I make the same mistake when jumping into older carburetted vehicles that have been converted, but can assure you that it is well worth developing the habit of turning the ignition on, waiting for the hiss to stop and only then cranking the engine over. It really does start quicker and save a lot of wear and tear!

12a. LPG Starting Procedure

(Petrol injected engines with single point closed loop conversion.)

It would be impossible for me to count the times I hear this statement -
"It doesn't start very well on gas......"

It is one of the most meaningless things anyone can say about a converted vehicle.

For clarity, most systems fitted in Britain NEVER start on gas, they start on Petrol every single time. It does not matter whether the fuel choice switch is set to the Petrol or Gas position, the system will only switch over to gas from Petrol if gas is selected and the changeover criteria are met (normally engine RPM reducing from 15 - 1600 and engine coolant temp. of 30C or so). When these facts are considered it is plain that no converted engine actually starts on gas (unless the changeover requirements are modified or the system is cheated with and emergency start procedure).

As for "It doesn't start very well on gas......" here is what really happens -

If the engine is stopped from gas running some unused gas will remain in the inlet manifold. When the engine is cranked for restarting with the switch in the gas position PETROL will be injected, but the remaining gas will also be inducted. Both fuels will be drawn into the cylinders, making the Fuel / Air mixture too rich for the engine to start.

This is only a temporary problem, further cranking will lean the mixture back down to Petrol only, the engine will start and a subsequent switch over to gas will occur if and when 15 - 1600 RPM are attained and then reduced.

If this slower start (actually the result of Dual fuelling) is to be avoided there is one very simple remedy - If you have made a journey running on Gas, Switch back to Petrol and run for one minute before stopping the engine. Dual fuelling will then be avoided and normal Petrol starting will be regained.

For the trouble shooter, once this passage has been read and thoroughly understood it will also clear up some other misconceptions about converted vehicles.

As an example, if someone says that their car will not start on Gas or Petrol, it will be clear that the vehicle has a basic problem with ordinary Petrol operation and its inability to start is nothing to do with any gas conversion fitted (as long as all inlet air pipes and inlet manifold connections are good).

13. Adequate Maintenance is Crucial....

It isn't only your ignition components that must be kept up to scratch....

A recent occurrence ( 5 / 10 / 02 ) gave us and one of our customers a unique but clear reminder of how important it is to keep on top of routine maintenance, especially on an vehicle that is converted to run on LPG.

Here's what happened -

The customer had asked for a re-tune of his LPG system, as the engine had all but lost its ability to idle on LPG under any conditions. Naturally we were interested as to why this should happen, the conversion was a long established and proven one, the car having covered several thousands of miles without a hitch.

The engine was run on LPG as slowly a possible without stalling and the gas analyser connected. It told us that the idle mixture was very rich. After some considerable adjustment, an acceptable idle was established and the fine mixture adjustment done. We couldn't think of any reason why the idle mixture should be so far out of adjustment - The vaporiser had undergone its initial 'bedding in' a long time ago and should not vary, especially to such a degree (two and a half turns of the mixture screw were required to get satisfactory readings on the gas analyser). We soon found out......

The engine was 'revved' a couple of times to make sure it performed OK before a test drive, but the second time the accelerator was pressed the engine backfired, blowing a large hole in the side of the airbox. Strange.....This car had not one single backfire since conversion.

We restarted the engine to find that again it refused to idle. Reference to the gas analyser showed clearly that the idle mixture was again well out, but this time it was too weak. Two and a half turns (back to the original idle mixture adjustment) brought the right figures back onto the face of the gas analyser.

So, what explains all of the events above?

It's pretty simple when all of the facts are considered....

The owner had earlier completed a 2000 mile summer trip to the South of France and back, in nice hot weather and dusty conditions...The air filter was so badly blocked by dust that it had severely restricted the air intake. This made the LPG idle mixture artificially rich as the air supply was reduced. When the idle mixture was leaned off because it had appeared too rich, it was actually ridiculously weak, to the point that it caused the backfire on revving the engine. When the hole was blown in the airbox the original (correct) air supply was restored, making the readjusted idle mixture show up for what it really was, very weak. When the idle adjustment was returned to its original (pre adjustment) value, all was well again.

This little account of events serves to prove how important it is to make sure that your air filter is checked regularly, and to show us that large changes in LPG performance often have a cause outside of the system itself.

Finally, note that the engine performed acceptably on petrol, which appeared to suggest that the LPG system was at fault - Nothing could have been further from the truth.

In a petrol injection system, the airmass (often called 'airflow') meter would detect that the air inducted was reduced by the blocked air filter, sending an appropriate signal to the engine mangement system (ECU). The petrol injected would have been reduced as a result, and the engine may have appeared to run as normal, especially as it is a 4 litre unit and has a lot of excess power. The LPG system on this car was not of a kind that gets airmass information from the airmass meter and thus it could not have done anything to reduce the amount of fuel it was delivering. This made the overall mixture apparentley too rich, most noticeably manifested in poor idle performance.

Whilst petrol injection is not directly dependant on the amount of air inducted, any inductive gas mixer system (closed or open loop) is, and if that airflow is reduced, the LPG mixture will appear to be too rich.

If there is a moral to this story, it is that whilst the LPG system may appear to be at fault at first glance, it may well be a problem caused by something else entirely.

It is not always the gas system that is at fault!

Another common misconception arises almost daily when we hear folk speak of their apparent problems with LPG operation - "It doesn't backfire on petrol but it does on LPG". The conclusion that the LPG system is at fault is then drawn, apparently a reasonable assumption to make.

However, this must be Red Herring No 1 when considering backfiring.

Recently we saw a car that later proved to be misfiring due to a faulty flywheel sensor. The fault went completely undetected on petrol because it cannot result in backfiring. There is nothing to ignite and explode in the manifold and plenum when running on petrol, only air. A stray spark will do nothing there.

BUT! If the manifold is full of gas/air mix (as it is when the conversion is one that uses a single inductive mixer) ......Well, we all know how that ends!

There simply is no point in thinking that if backfiring does not happen on petrol, the fault must lie with LPG. Yes, LPG is present, but something has to ignite it.

In cases like this, LPG is the victim. Sadly, it draws our attention to hidden faults by making a loud noise, showering us with bits and looking guilty!

An interesting footnote for those considering a multi-point sequential gas injection conversion (SGi) is that neither of the above problems can result in backfiring when using this system - The potential for large scale backfiring is practically eradicated by true SGi.

14. Long term effects of 'machine gunning' during filling

It is not uncommon for multi valves to make the 'machine gunning' sound during filling, normally when the tank is almost full.

It is not dangerous in itself as the moving parts are non spark generating, and I have observed many owners filling through (and past) the sound to get a few litres more gas in the tank.

Yesterday we saw a long term effect of this practice - The car was brought to us leaking gas from the undervent when filling was attempted. A short investigation showed that the filler pipe had snapped clean off right next to the tube nut on the multivalve. Closer inspection reveled the almost crystalline appearance of the ends of the copper tube, suggesting that the fracture and eventual complete failure was due to work hardening of the metal.

The only source of vibration sufficient to cause this problem is thought to be the owner's practice of allowing the valve to 'machine gun' to get as much in the LPG tank as possible. The pipe was replaced and normal filling resumed. We advised the owner to stop filling when the noise (vibration) begins, and not to fill through it.

An interesting footnote is that the latest OMVL multivalves do not 'machine gun' in this manner and the problem is completely eradicated.

Go LPG only install OMVL multivalves as a matter of course.

15. Cars that smell of gas.....

One of the most common problems we are asked to investigate is just this.....

"It smells of gas......"

The first thing we do is to check for leaks using a gas detector, but in a high percentage of cases no leak is found. The cause is often simple, but not widely understood.

To provide a quick guide before you read on, here are some basic facts about LPG.

1. LPG does not smell at all.
It has a smell added to warn us of its presence. The Smell ( provided by a 'Stenching Agent' ) DOES NOT BURN. Often, the smell of exhaust fumes can smell just as if gas is present, when, in fact, all the gas has been burned by the engine and only the stenching agent remains. This can result in unwarranted concern.

2. The Smell of Stenching agent
can remain on your hands after filling for a very long time, even if you washed them once or twice. Also note that the smell of any released gas can 'hang' for days on any item it has contacted, if not washed away. A good example of this is a faulty vaporiser that we removed from a car but did not throw away. 6 months later it still Smelled of gas. This persistent smell can also result in unwarranted concern.

LPG 'Leakdown'.

Vehicles with single point mixer systems -

When the vehicle's engine is switched off from gas operation, some gas will naturally be present in the induction system and remain unused. The gas is heavier than air and will tend to 'sink' to the lowest level possible. If the pipe from the inlet manifold (where the gas mixer is often sited) is sloping downward the remaining gas will seep down into the air filter and may well escape to atmosphere, resulting in the smell.

This effect is called 'Gas leakdown'. It is not a dangerous condition as the gas is in low concentration, further reducing as it mixes with the surrounding air. The next time the engine is started, no gas will leak down as it is drawn into the engine, but the next time it is stopped identical condtions will arise and the smell may reappear. It would not be unreasonable for a driver to imagine that the smell is present when the car is running, giving the impression that the system is leaking all of the time.

Leakdown may be further complicated. I have seen at least one car (Ford Granada Mk 3) where the sinking gas was able to find its way into the intake of the fresh air system. The car's ventilator fan had been left on to demist the screen, but then turned off with the ignition. The next time the engine was started (on gas or petrol) the fan came into operation and the gas effectively stored in the fresh air duct was blown into the car. (The cure for this would be to resite the fan's air intake away from leakdown.)

Curing leakdown......

There is a cure for leakdown, but sadly most customers press for the cheapest conversion possible and it is not included in their system.

A timer device is available which wires into the car's ignition system and the gas solenoids. When the ignition is turned off, the gas is turned off by cutting power to the solenoids, but the engine will continue to run until all the gas present in the induction system is used up. No smell of gas can then occur as no leakdown is possible. Simple, effective, but almost universally ignored.

There is a cheap alternative cure which is to always switch back to Petrol and allow the engine to idle for a minute or so before stopping it. No gas can then be left in the induction system and leakdown is zero.

Multi Point Gas Injection Systems

These can also display leakdown, but to a much lesser extent. There is far less gas in a shorter length of the induction system. As a result, the potential for leakdown is greatly reduced and often goes unnnoticed altogether.


1. Most reported'Smells of Gas' are not dangerous, and do not indicate that there is a problem with the gas system or its installation.

Other factors can be to blame......

An owner of a car converted elswhere complained that his car smelled of gas every time he filled the tank up. A long investigation was carried out but no leak or cause was found. Only when the customer later mentioned that he could smell gas when he filled his other vehicle up with Diesel did the penny drop....

This Customer used the same gloves to fill his first vehicle with LPG and the second with Diesel.
The smell of gas was on the gloves.....

2. In the case of leakdown the effect can be cured by asking the installer to fit timed shutdown equipment or the driver simply making sure than the engine is always run down on Petrol for at least one minute before stopping it.

Some other facts

People running Petrol cars have become accustomed to a slight smell of Petrol being present if the bonnet is opened. Likewise, those running Diesels are not the slightest bit surprised if they smell Diesel when they open the bonnet. Both are most likely to be the effect of fuel 'leakdown' from the induction system but are ignored as being normal.

However, if the vehicle is converted to run on LPG, a new smell of gas (actually the 'Stenching Agent') can cause great alarm, but indicate no increase in risk.

And finally...... Don't be TOO comforted by this explanation and ignore ALL 'Smells of Gas'!

If you think you may have a leak, DO go and get it checked out (we'll do it for free) and make sure no serious condition exists.

Just be a slightly more ready to accept that sometimes a whiff or two of gas smell does not indicate that your gas system has been badly fitted by your installer, nor that it is in a dangerous condition now.

16. Servicing Amomalies.

Although we have said that ordinary servicing of your converted car is no different (save spark plug type and setting), it is important that the subtleties are observed.

We have seen several cases of cars misbehaving 12 months or so after conversion. In almost every case the car had just been serviced and this was the direct cause of the problem.

Spark plug gaps for LPG are normally less than those for Petrol operation. If this goes unnoticed by the servicing garage it may well cause problems.

In one extreme case we had a complaint about a Ford Galaxy which had covered 27,000 miles after its LPG conversion 9 months previous. The car had recently been serviced by the owner's normal company and begun to give trouble. At low speeds in high gears (labouring the engine) the owner said that it would backfire almost every time. It was interesting to see why he was in the poor driving habit of labouring the engine. He did it whilst steering with his right hand and holding his mobile phone to his ear with the left hand. He had no hands left to change gear......

Anyhow, the fault (after some 8 hours of fault finding) was traced to spark plug HT leads on two of the four spark plugs. They had been pushed on to the plug tops but not turned and locked (as one must do on the 2 litre Galaxy). When the engine shook during labouring the HT lead caps shook too, temporarily breaking their connection. This interrmittent connection could only have one outcome...... Backfiring.

The fault was cured instantly by fitting new leads and securing them to the top plug electrodes properly. The owner was dismayed to find that this was not an LPG warranty issue and therefore our time was chargeable.........

Sometimes you can't please anyone!

17. Refusal to run on gas in hot weather.

In the Summer months it is possible for the liquid gas held in the system (especially that held the front to back fuel pipe) to be heated and vaporise, causing a high pressure vapour lock. When a car is left hot and the engine is restarted after some time it may refuse to run on gas and stall upon changeover. This happens because the vaporiser is being denied a supply of liquid gas (which it relies upon) and when the engine stalls the tank solenoids are closed again, preventing the vapour from being bled out of the line.

The cure is to bleed the gas line by starting and changing over to gas as many times as it takes before the vaporiser's liquid supply is restored and the engine will run on gas.

The phenomenon is found in some cars more than others, definately more prevalent when the car is left parked on black (or dark) Tarmac surfaces and often manifested if the car is parked at the beginning of the hottest part of the day and restarted before the air begins to cool in the evening or during rain.

17a. Tanks that will not allow you to 'top up'

Following the theme of hot weather and its effect on liquid gas, it is possible to fill your tank in cool condtions, use 10 litres or so in running but be later unable to 'top up' the tank for a longer journey if conditions have become much hotter. This is simply becuase the tank has warmed up and the pressure within it has begun to rise. The supplying LPG pump cannot then overcome the pressure of the vehicle's tank and it is impossbile to put any more gas in it.

The solution here is to use the car anyway, run the tank low and fill up when conditions (and the tank) have cooled down. It will then be possible to refill the tank as normal.

17b. SGi systems switching back to petrol in cold weather.

Whilst we are on the subject of LPG storage in your tank, and have established that tank pressure rises with an increase in temperature, consider the opposite effect.

When temperature decreases, so will the pressure of the gas in your tank.

If the tank is very cold (0 degrees C and below) the tank pressure may well drop to a level that will cause some of of the more 'twitchy' SGi systems to switch back to petrol. They are programmed to do this when tank pressure is low. It is done because it indicates that the tank is nearly empty during more normal weather. You may think that this is a fault with your SGi system. You might later find that it will stay on gas all day in warmer weather, and not switch back to petrol at all. Perhaps you may think that it has 'fixed itself'.

It hasn't, and it isn't malfunctioning either. It is just doing what it is told to do by its maker, switching back to petrol when tank pressure is low.

This problem is much more likely to occur in high performance converted cars or cars with very large engines as they demand more LPG from the tank in less time, requiring a higher tank pressure which cannot be maintained in very cold temperatures.

A larger diameter fuel pipe will reduce the effect because more gas (by volume) can flow through a larger pipe at the same speed as less gas would flow through a smaller pipe. Heating the LPG tank would also bring benefits but that isn't an option.

Converters in general are a clever lot but they cannot change the laws of Physics, nor the rules on LPG installation in this Country. We are not allowed to heat the LPG tank! It is one of those things that you'll have to allow for on cold days, if your vehicle and gas system is susceptible to this effect.

18. Variations in how much LPG an 'empty' tank will take when filled.

Is the tank really empty?

There are many factors to consider here but if anything can be said to be utterly true, filling an LPG tank is not even similar to filling a petrol tank. With a Petrol tank of 12 gallons (for instance) you will know well when it has run dry (ignoring a small amount of unusable petrol remaining) because the engine will stop. In the case of earlier type LPG systems your engine may begin to 'die' when the LPG tank is low on content but this does not mean it is empty. It may even run the engine again the next day, even if no more gas is put in. This is due to the recovery effect of tank pressure and variations in temperature.

To make things even more confusing, most modern SGi systems are progammed to switch back to petrol automatically when LPG tank pressure falls below a pre-set limit. This does NOT mean that the LPG tank is actually empty when the system switches to petrol and the alarm sounds. The alarm indicates low tank pressure as opposed to low tank content. As an example, a tank with a lower temperature will give lower gas pressure than a warmer one. Thus, if we drive during reducing temperature conditions it may seem as if the tank is empty, when in truth the tank's pressure has dropped because the tank got colder. You can see this first hand if getting an alarm sound for low pressure when driving on a cold night, only to see the system run for miles more on a warmer day afterward.

Going back to the example 12 Gallon petrol tank, you will be accustomed to always getting very close to 12 Gallons into it when filling it from empty as petrol does not change its volume very much with temperature changes. LPG however, changes its volume at 10 times the rate of petrol for any given temperature change. In brief, you will get a lot less LPG in your tank on a hot day than you would on a cold night.

Other factors must be considered;

Tank fill cut-off devices.

There is a cut-off valve in your LPG tank operated by a float/arm arrangement. Another is fitted to the filling pump restricting the amount of pressure that your tank may be filled to. If you are using the cut-off to judge the tank 'full' (actually 80% of its water capacity) then which cut-off device stopped the filling?

The Tank filling limit device (part of your conversion).

This works just like a lavatory cistern and relies on being level. If the car is not sitting level it will naturally result in overfill one way and underfilling other ways. Note that toroidal or 'filled toroidal' (spare wheel) tanks are much more susceptible to this effect than cylinder tanks are.

Pre-set filling pump pressure.

This will vary from pump to pump and day to day due to temperature of the tank and its contents.

Also note that filling pump pressures vary as their contents reduce. An almost empty filling tank will not give the same 'fill' of your tank as one that is almost full. No one can tell how much LPG is actually in the filling tank unless you just saw them fill it up.

The Filling pump automatic cut-off pressures will also vary pump to pump.

If you want to get the greatest amount of LPG you can get in your tank, arrange things to your best advantage;

Use a filling Station that has a full or near full tank.
Use a filling pump that delivers higher end pressure (it can take some time to find that out by comparing it to others).
Always stand your car on level ground.
Fill up during cooler conditions (say late at night in summer).

19. OBD reported 'Misfire' trouble codes for one or more cylinders.

These are so often misunderstood that the subject is worth covering, along with the root cause of the confusion. In the main, it is an area where language lets us down.

What do you understand by the word 'Misfire'?

Does it mean that a cylinder hasn't fired at all, a 'Missed Firing?'
Does it mean that a cylinder has fired, but not in the desired way, a 'Misfire'?

In ordinary speech we take it to mean a failure to fire, the engine or a cylinder is said to be 'misfiring' , although we actually mean it has not fired at all. An 8 cylinder engine running on only 7 cylinders will often be said to be 'misfiring' although it is actually failing to fire on one cylinder. The failure to fire results in power loss and engine imbalance which is what can be felt when driving a car with this problem. Clearly, it would not be correct to call this condition 'Misfire.'

'Misfiring', when reported in OBD terms, means something quite diffrerent to a failure to fire. It means that the cylinder the trouble code relates to has fired, but the result of the firing (combustion) was out of parameter. This is why we don't feel a reported misfire when driving. The cylinder involved hasn't failed to fire, although it may be firing out of parameter because the resulting combustion is incorrect.

The number 1 and 2 causes of true misfiring ( in the OBD sense as described above) are spark plugs and coils. This happens because incorrect ignition will often leave some fuel unburnt and the engine management system will see that cylinder as running too rich as a result. If this isn't the case, a reported misfire in OBD terms can also mean that the fuelling of the cylinder is out of parameter which could be due to incorrect amounts of fuel being injected. A cylinder that is running with fuel too rich or too lean is Misfiring.

The path to cure will be a process of elimination, but it is always best to begin with the cheapest and easiest things, the spark plugs. Replacing those will not be money wasted, especially if you cannot remember when the car had the last set fitted (see point 5b). If the OBD misfire codes go away then that's great, but if they don't, you'll have done some useful maintenance and will have ruled out the spark plugs for a beginning.

Just to complicate things and hamper diagnosis, some modern cars (Mercedes and Citroen for instance) will actually shut down a truly misfiring cylinder which can be misleading when trying to diagnose the problem. The telling factor here is that an engine restart will bring a cylinder that has been shut down back into operation, until the misfire occurs again. If that is happening in an LPG converted car with (say) an ignition fault only present when running on LPG (see point 5 above) then things can become really difficult to fathom. Unravelling all this is easier when things are broken down, the aim being to idenfity the cause of the reported 'Misfire' trouble code. As with many things, it helps to know what the trouble code is actually telling you.

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