P38 4.0 LPG - Misfire Under Load on Petrol and Gas

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P38BORDEAUX

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Hi all

I have seen a few similar posts about this but none seem to match the problem i am having.

I am running a 4.0 2001 P38 with an LPG conversion.

When idling or driving gently it runs fine but as soon as you give it a little more on the accelerator i am getting air popping and banging coming back into the airbox and i am down on power.

I started to experience a misfire so took it in to have it plugged into a diagnostics computer, this picked up that 6 of the cylinders had a misfire. It is running nearly new magnecor leads and i have fitted in the last 4 months NGK LPG1 plugs. Therefore we thought it might be the coil packs, both have been changed still experiencing the same problem.

The problem is on both petrol and LPG and happens when the engine is hot or cold.

I have seen a few posts the suggest it could be crank sensor, but i would have thought this would show up when plugged into the computer.

Also i know LPG creates more condensation than petrol could this cause the cats to block up and cause back pressure?

Can anyone offer any advice as i am really in the dark on this.

Any help is greatly appreciated.
 
:welcome2:

LPG shouldn't block anything - it burns cleaner than petrol, so you'll find your engine and plugs, oil etc are infact cleaner because of it....

It does create more water vapour in the exhaust (can be seen on cold mornings more evidently due to the dewpoint and humidity levels) but this could not block an exhaust system - the Cats should run hot enough to boil and condensated vapour almost immediatly.

My thoughts are it could be the MAF sensor, I assume it is a sequential LPG system and not a single point - as the Multipoint will use the MAF data for LPG injection - if it is single point, it could have backfired through the induction pipework damaging the MAF, or split the induction pipe upstream of the MAF causing an air leak!
 
Hi

What do you mean by sequential LPG system?

So does the MAF sensor get affected when running on both petrol and gas?

Is there anyway i can test the MAF sensor? Will removing it completely give me any clues?

Thanks
 
Last edited:
A multipoint/Sequential system uses individual injectors for each cylinder, and injects LPG directly into the intake for the cylinder.

A single point system uses a ring on the air intake and squirts the LPG into the whole air intake and then gets drawn into the cylinder when the intake valve opens...

The problem with single point systems is they are not as efficent as the multipont version and also if the engine revs are too low, or the gas pressure is too low, it can back fire through the intake ducting with a pop and bang which can damage the MAF.

The Multipoint system can use the Mass Air Flow sensor to measure the incoming air and inject the required amount of gas to run correctly....

If the MAF is dicky, the LPG system and the Fuel system will be getting incorrect information causing it to under or over fuel with the inherient running problems that will ensue.

Start the car, then disconnect the MAF, if it runs the same MAF is duff, if it runs worse MAF is good....
 
Ahh I have sequential in that case.

Ok will disconnect the MAF tonight and see what happens. Will feed back tomorrow with the result.

Thanks for taking the time to help.
 
Hi Datatek

No not from Bordeaux. It is a special addition P38 at the end of the P38 range, they filled it with toys and cream leather with red piping and red carpets.
 
Took the plug off the MAF while the engine was running revs dropped slightly then picked back up and runs the same.

So looks like i need a new MAF will buy a new one and feedback.

Thanks for the help on this.
 
Update on progress.

Not the MAF, going to try a new crank sensor next.

Diagnostics plug in today just showed a misfire but no fault codes we could match.
 
Ok update

def not the MAF, changed the crank sensor no change.

Took the 2,4,6,8 rocker cover off as this seemed to be the side that was making the most noise, nothing out of the ordinary.

Had a specialist plug it in to their diagnostics, showing misfire on 5 and 8.

The suggested either cam or valves.

Problem has been getting slowly worse and now undriveable, seems worse when the engine is cold, popping and banging back into the airbox. I have noticed that when you floor it you get the normal misfire sound and well down on power but does not pop and bang back into the airbox.

Any ideas? Could it be cam etc, after initially thinking it was electrical i am not sure now and poss think it is mechanical.
 
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