5.2 V10 Misfire

A8 All/ S8 All
#1

I am looking for some guidance on where to go next on a misfire on my car,

The misfire is noticeable more on a cold startup but can also be felt at idle and part throttle up to about 1800rpm after that it can’t be felt or seen in misfire counters in VCDS.

The misfire is not serious enough yet to store a fault or put on the MIL

So far I have removed, inspected all the plugs and found one of them to be sooty cylinder (2)

Have replaced all the plugs with new ones and moved the coil from the cylinder (1) with the misfire to another cylinder.

I had a friend do a smoke test on the inlet to see if there was any leaks and there were none.

Also inspected the inlet manifold for broken flaps, they are fine.

With all the above done I still have a misfire on Cyl (1) with occasional misfire shown on 6 which i suspect is a false positive to do with harmonics of the misfire from Cyl (1) just a guess ::slight_smile:

The only other diagnostic info I can provide at present is the fuel trims are about 4% negative which if I understand correctly means the car is running rich and its trimming back to bring it the correct AFR?

Would love to hear your thoughts on where to look next?

#2

Sorry forgot to mention I also checked the compression on the car and all cylinders are at 12bar

#3

Most of the time the cars missfire on cold start due to the SAI system and the build up of carbon.

Carbon would be the biggest thing you really need to look into and remove. Carbon will take out injectors and cause the car to run rough.

As for the missfires cylinder 1 is on one bank and cylinder 6 is on the other side. missfires are detected by the 02 sensors so its a true missfire it wouldnt be a false positive.

The fuel trim isnt anything to consider right now. The car has no rich or lean codes. The fuel trims will almost always be adjusting as weather temp altitude and fuel quality all play roles.

If you dont have any outstanding codes Id really make sure to look into how bad the carbon build up is.

#4

Thanks

I am going to go digging deeper and look at having the injectors out and cleaned and decoke the inlet valves.

As for the misfire recognition I always thought it was monitored by the knock sensors, and my thoughts about the false positive was due to the fact cylinder 1 and 6 share the same crank pin?

#5

No the knock sensors pick up increase cylinder noise or knock. So a missfire would be a quieter fire. The 02s are wide band so they pick up the huge change in air fuel mixture given the last cylinder burn cycle.

Checking the carbon and the injectors would be the usual suspect and a needed service on these cars anyways.