Help with S6 (C5) 4.2 V8 slight missfire.

[Quattro Dan.]

I have a S6 C5 4.2 V8 2000 year, AQJ engine code. I have been trying to trace and fix a slight misfire on idle without much success, any pointer would be appreciated.

So far I have fitted new spark plugs (Bosch), tested and fixed small bore vac pipes. Tested Kombi valves operate.

The car drives fine without any hint of hesitation or misfire. However, on idle the engine very slightly shudders and I can hear a misfire, it seems to be amplified down the airbox.

I think the ECU in my car is slightly too old to use VAGCOM to log misfires?

Any idea how I can test the coilpacks? Also both lambda sensors look to be original (117,000 miles now)

MAF is about 2 years old.

Thanks in advance.

[Ryss]

Do a compression test. Sounds like a burnt inlet valve, where the gas is bypassing into the inlet system.

[Quattro Dan.]

Do a compression test. Sounds like a burnt inlet valve, where the gas is bypassing into the inlet system.

Thanks for the suggestion, I suspect you are correct.

Cheers.

[AndyP]

Thanks for the suggestion, I suspect you are correct.

Cheers.

Want to borrow my bore scope camera to have a look?

[Quattro Dan.]

Thanks for the kind offer Andy P. but I have done a compression test and leak down test today and all seems well

This leads me to believe the fault lies with either a coil pack, injector or Lambda sensor?

The Lambdas look original and are on 117000 so I will swap these for new tomorrow.

Not sure how to find which if any coil pack is slightly at fault (remember it drives fine, its just a slight miss at idle). I may buy 1 new one and try it on each cylinder one at a time.

Injectors will be the last port of call if the above doesn't resolve the issue.

Cheers.