S6 V10 Misfires on Cylinder 6 only on coldstarts and really random. DESPERATELY NEED HELP!

A6 All/ S6 All
#1

Hello guys, my first ever post here on this page and I’m glad that there are so many well educated V10 enthusiasts around here!
To my problem: I drive a 06 Avant S6 with 126k miles on the original engine. Since I have the car which is more than a year now I’m chasing the issue of having P0308 (or so) issues of misfires on cylinder 6 during some cold starts. The issue is very random. The car will start flawlessly for 2 or 3 weeks straight, then start like crap for one or two days. No issues during driving or anything. After 20 meters of driving it will run on all 10 cylinders again.
So far I have changed following parts:

  • Carbon cleaned it.
  • intake manifold
  • brand new NGK sparkplug
  • brand new NGK coils
  • 2 new throttle bodies
    -valve cover gaskets
  • oil separator
  • all 10 injectors; brand new Hitachi ones Since one died on my on the autobahn which was an unpleasant experience!
  • compression test on Bank 2 was between 10.5-11.5 bar which is within Audi specs
  • I don’t burn oil, however my upper oil sump leaks since day one which is an engine out job to fix and its just not worth the hassle.
    BIG TURNING POINT:
    In February this year I’ve replaced my battery since the old one just died and right after the new one came in the issue stopped for 2 months straight! Not a single misfire!

I’ve already spent 11k € on the car and I’m really desperate considering I’m still a college student and everything I earn is put into the car which is my total dream car.

Any tips are welcome!! Please don’t let my dream turn into a nightmare!

#2

Hi there!

Have you tried cleaning the coil pack connector? Or have you checked it’s wires? This engine needs to have very brutal misfire to be noticed by ECU in my expirience. I would check spark plugs state, becouse you may have an issue with more cylinders, and for some reason #6 is the only one noticed by ECU.

#3

My Mechanic and I checked them and there’s nothing out of the ordinary with the connectors. I once hooked up VCDS while performing a coldstart and it only showed #6 misfiring. Once I drove for 15-20 meters it stopped completely. No other codes except that and the broken engine mounts on both sides.
Yes it’s a harsh misfire the CEL comes on and stays on. But besides the misfire there isn’t any code.

#4

You mentioned a compression test but only cylinder 2 what about the other cylinders? When you replaced the battery did you happen to code the new one in VCDS?

#5

Cylinders 6-10 were measured all between 10.5 and 11.5 Bar, so within Audi specs and no big difference in compression.
I cannot remember whether my mechanic coded the battery in or not all I can say now is that sometimes my battery light stays on when the coldstart is crap, but not every time.

#6

Never had any issues with cylinders 1-5 except when the injector died but this has been fixed.

#7

If only at cold start, it can be linked to some humidity in the connector / wire… Have you inspected the cylinder with a camera to see the walls ? It shouldn’t be that due to your good compressions, but it should not hurt to check.

#8

Haven’t done that yet but usually bad cylinder walls often come with metallic scraping, excessive oil consumption etc. Which I don’t have at all. Considered that theory with my mechanic as well but this would’ve meant that EVERY cold start would be bad right? It’s always just one or two days within a three week interval.

#9

And (this is by far not a scientific study) by my own experience these damages start between 20k and 70k miles usually. Often due to owners neglecting oil change intervals and ignoring driving it in. But mine according to audi has been well maintained and is way past that threshold.

#10

I recently had a similar problem with random misfires. For me, the problem seemed to disappear after:

#11

Thank you I will try that! I’ve had multiple occasions where the battery light would flicker for a second 90% of the time it misfired.

#12
#13

UPDATE: checked the sparkplug and it looked very good, not even remotely caked up. Issue kinda feels like a having MAF problem (RPM going up and down) also I’m having an appointment for an endoscopy next week which hopefully does not reveal something bad.
But the typical symptoms are not shown:
-no excessive oil consumption
-no loss in compression

  • sparkplug in good shape and condition

Any other things you guys recommend checking in the meantime? Gonna use all your advice this weekend!

#14

I have some misfires. My theory is that I have an air leak.

When I cranked down on the IM bolts I could reach, the frequency seemed to lower. I cannot reach the ones under the throttle body without pulling it, and I believe I still may have an air leak in the throttle body/plenum area that may be contributing to it.

I haven’t had a chance to prove this, but the misfires are not causing any performance issues, since like you, they go away when warm and fortunately when cold, they are minor.

Maybe you can use a smoke stick to see if any is being drawn in around the IM or Throttle plenum areas.

Ed

#15

My original IM developed leaks at all 4 corners. We discovered it by pressurizing the IM and applying a soapy solution to the exterior of the IM. Unfortunately, that required replacement.

#16

Just to follow up—misfires came back. So the battery is not the only problem

#17

Is it during cold starts only or also during driving? Have you done an endoscopy of the cylinder walls?

#18

It’s just random/occasional misfires at idle. Mostly happens in bank2. I swapped a coil pack between banks and the faults did not move.

If I reset the codes while the engine is warm, I can drive aggressively for hours (2lane mountain passes) and the engine is perfect.

#19

Does the CEL flash? How are the misfires felt? Like aggressively or just a slight tickling? If it’s a tickling it might be a leak within the PCV system.
But before you go ahead and starting buying parts and changing them I’d take a look at the cylinder walls nevertheless. I looked at mine and saw the alusil coating peeling off on all cylinders which is the fate of every FSI V-engine. Seen 3.2s and 4.2s doing it too.

#20

When it misfires, It’s usually just a rough idle with the CEL flashing.

Also, I recently replaced the oil separator and that seemed to fix the occasional P0171/0174 system too lean codes. But I did note that the membrane of the old part was still intact.