Misfire... continual problem

[1781cc]

OK fellow owners, I am pulling my hair out at the moment trying to sort a misfire problem that is driving me up the wall and all the money out of my bank account.

its an odd one to describe, the car (a B6 S4) has been LPG converted, but this problem affects both Petrol and Gas, so, to rule the gas system out I have been running on BP's finest for over a month.

The problem, when the car is fine it drives perfectly, but once its warmed up there is a hesitancy and slight juddering in the power delivery as I put it under load, general driving is affected but less so than heavy load, so if cruising at 2-3K rpm is ok, but acelerate and the power comes through but its far from smooth and clearly misfiring.

so, I replaced all the coil packs at a cost of £180, the car has been inspected previously for ecu problems and nothing. the pre-cats have been gutted but not yet mapped, I am guessing that this shouldn't affect things that much because I had the misfire before.

so, what to check next? sparkplugs? or is it something related to the temperature sensor? I have heard things about that in various parts? if it is neither of the above, what else could it be? where should I start looking?

any advise welcome as long as it's on topic and not about eating yellow snow...

seriously, I can't tell you how frustrating this is for me...

[Contigo]

Does the car not throw any fault codes?

[1781cc]

it has thrown a couple, N31 I believe, circuit malfunction...

only problem is I upgraded to windows 7 yesterday and my VAG-COM wont work on it now...

[verysideways]

Could it be an airleak in the intake system? For example a split intake pipe between airbox and inlet manifold (thus throwing the fuelling out slightly)?

[1781cc]

Not sure, I didn't find anything and I tried a new mass airflow but made no difference, so I returned it. It's such an odd thing to trace, map maybe?

[xyber]

Cam sensor getting a bad signal or just dirty? Plugs just to rule them out

[elmo]

I was going to suggest the MAF, but I see that you have already ruled this out.

Have you had a compression test done on all of the cylinders? just in case it is a mis-seated valve.

I would also get the injectors checked out, but without you VAG-COM this may be difficult.

Plugs are an easy option, and worth a try for only £50.

[1781cc]

I had the compression test done when I was tracing the LPG problem. all fine.

I don't think it will be an injector as the problem is common to both running on LPG and Petrol and they use different injectors, the misfire is consistent in both cases.

only problem now is I cant get the bonnet open no matter how hard I pull on the cockpit lever. any ideas on that first? lol **shaking head in disbelief at the grief this car is giving me**

[Greg66]

I've had spark plugs go once on me, in a Mk II 16v Golf. As I remember, the car was fine 99% of the time, but if you floored it, the rev needle would hare up though 5000, 6000, but then instead of climbing to 7000, it would get to about 6500 and then start dropping back, with a corresponding power loss, even though right foot remained planted. Almost as if the engine was being asphyxiated.

Fit your symptoms?

[1781cc]

something like that mate, yes, I suspect an electrode has gone in one and when they get up to temperature things expand and thats when it misfires. in the end, the misfire, coupled to the bonnet problem (where they have to cut through the grill) has meant that I booked it into Inde-Tech in Milton Keynes (who have always been good with the car) and told them to sort it, just keep the car until its done (not open ended, but fit it around other jobs and let me know)

I have had just about all I can take at this stage...