Hi everyone I'm new here and running in circles with what to do. I'm trying to help my cousin out with his Audi due to him not having the finances of the dealership. He has a 99 Audi A6 Quattro with a random missfire on cylinder 4,5,6. Missfire on 4, and same with 5, and same with 6. I don't remember the codes but that's the basic of it. Now at idle it is rough very rough and of course the CEl flashes to for the missfire but past 1200 rpms it is fine no missfire. And it also drives fine. But once you make a complete stop or drop your rpms lower it does it. I installed new coil packs( mounted on front of engine as a set) wires and plugs. Then I also informed him his crank case breather line that goes to the valve cover is cracked in multiple areas. If anyone has a suggestion or guidance please help. I don't have the vag com to fallow it. I don't ever mess with audis or Vw tbh. I've seen forums about the icms or cats. Or injectors. But I want to see if anyone has delt with this exact issue I've searched the forums and haven't see any post about it. Thank you

 

If the PCV plumbing is cracked, it's causing a vacuum leak. Just about every time it happens on my D2 A8, it ends up glazing a spark plug or two causing a misfire.

 

I figured that but wanted others opinions. It's ordered and will be here in a couple of days thankfully. No I also check the injectors to see if any of them were not firing but all seeme to be firing as the engine demands it.

 

A glazed spark plug or two will cause a misfire, especially on acceleration. The plug is basically grounding out (via a glazing on the ceramic insulator between center electrode and the outer metal housing) and producing a weak spark which isn't quite hot enough to fully ignite the fuel/air mix. I fought this issue on my A8 a few years ago and it drove me nuts until I figured it out.

 

That's the thing. It drives perfectly fine in my opinion and his. But once it sits to idle man it's rough. Lol

 

At idle with the throttle butterfly valve closed, the engine is pulling a LOT of vacuum in the intake. Any cracks or vacuum leaks will definitely introduce more air into the intake than it's supposed to. This will make running at idle a little rough sometimes. Fix that cracked PCV stuff and I bet it idles fine then.