a bit of background first, its a audi v6 we had in for service last week about 130k miles on it.

after the service it drove out fine. Anyway a couple of days ago it was recovered as it stalled on the owner. Upon arrival to where it stopped the car started and drove back with a small misfire at idle and low revs but heaps of power at high revs.

We got it back to the workshop and fitted some new plugs, then leads then i traced it down to no 1 & 5 cylinders. Checked and double checked firing order etc.

Fitted new coil pack and its still no different.

The reason i changed plugs and leads was because the leads were old and plugs had done 10k since last change but coils was then changed after realising you can remove no1 and 5 leads without affecting the engine.

These 2 plugs are soaking wet with fuel when removed asift they are not firing but the system fitted has 3 coils giving the spark to 2 cylinders at a time but the misfiring cylinders are both from different coils??

Im starting to think about injector issues, Are they staying open?? If so why? If not, why are the plugs so wet?? Think i will check the injectors with the low amps clamp monday.

Any ideas welcome!

Thanks in advance Brad
P.S it has codes relating to 02 b1 s1 and b2 s1 low activity as expected with a rich engine condition. src=https://cdn.boardhost.com/emoticons/grin.gif>

Hi.
Have you checked to see if there actually is a spark at 5+1 leads. I would check the signals to the coils from the ECU before the injectors. A 98 Audi should be able to fault a shorted injector and flag a fault code.
HTH
Fiachra.

Time to get the scope out and look at the secondary signals?

Mark

I would do a relative compression test with picoscope first.
Shane

I agree you should see a misfire code but there isnt.? Yes number 1 & 5 plugs are sparking very well altho they are very wet. I have swapped them around with no difference, checked and double checked firing order and even tried swapping the 2 misfiring cylinders with no joy in desperation.
If compresion was at fault surely it should still burn the fuel??
I only have the vantage to test with no pico.
Thanks for all the thoughts and suggestions, i keep thinking there is something simple at fault but cant find what.
regards Brad

I think you need to start with some basic tests rather than just changing parts without proving them faulty. I cannot see any mention of a compression test although it has been suggested. (Does not have to be electronic. You must have a compression gauge?).
Spark plugs may give a spark when out of the cylinder but that does not mean there is enough energy to produce a suitable spark under compressed conditions. A spark gap tester is better or ignition primary / secondary tests as already suggested.
If these tests eliminate those basic factors then you can proceed to air leaks / injector signals etc

Steve

Your vantage scope will test primary and secondary ignition good enough for you to compare different cylinders as any scope is better than no scope I was told.

Are you sure its not the injectors leakng under rail pressure.

I am with Steve with this. In todays age, people seem to be too quick to dig out the scope without going through the basics. Where are the exhaust emission figure? That may give you an indication of whether you are chasing an ignition, fuelling or mechanical problem. Depending on the results, you can then proceed down the correct diagnostic route and if necessary dig out the oscilloscope.

Regards

Pete M

I get what you are all saying,
I wanted to do a compression test on it but my boss told me straight that it cant be compression related as the plugs are so wet he says "it cant be firing, if it had no compression it would still burn the fuel"

Im starting to think either injectors or signals to the injectors??
I will have another crack in the morning.
Wish me luck!

>>> if it had no compression it would still burn the fuel <<<

Since when?

Yes i know what you are saying about the compression thing but if he sees me doing a compression test he will tell me im wasting my time (and his money)


Anyway a quick update,, it wouldnt start this morning , checked fuel supply and it didnt seem great so i checked the pump to find there was no fuel in it!!! nearly a quarter according to the gauge but nothing going out at the pump end.

Had to take it out to get on with other work and it still had a misfire but was idling and running better than before... fuel helps a lot eh ??

I would like to check fuel pressure as the pump is fairly noisy but having trouble rigging up my pressure guage. May need to fit it too the actual pump. Should i expect 3 - 3.5 bar on these?

Many thanks for all your help.