I have read a great many threads related to rough idle and related codes while trying to diagnose my problem.

My car is a 2003 A6 Q Sedan with the 2.7T engine. Important to note this car has only 46,000 miles.

One issue is that the Bentley manual and many comments in the forum mention the ignition control modules (ICM) on top of the airbox, but I do not have these at all. Could there be later and earlier versions? Or, since my car came from California could it be that a different system was in use there for emission control purposes?

During a cold start-up when running in open loop, all seems fine. Idle roughness increases with engine temp and by the time normal temp is reached it will jump from 1,000 rpm to 500 or slightly higher in a regular pattern. It may stall at that point too. When in gear, slight pressure on the throttle pedal against the brakes will steady the shaky idle, but it is still there.

I get the misfire codes P0300 and P0301 (cyl 1) and all of the other cylinders too. I get P0171 and P0174 which indicate lean running on each bank. I have also swwn P0570 recently indicating too high idle speed but not sure if that is because of my own actions with the gas pedal.

I have cleaned the MAS with the appropriate cleaned and also the throttle body. I have replaced some of the small vacuum lines because the woven sheeth on them looked ragged, even though I could not detect and leaks there.

I sprayed around all kinds of possible vacuum connections with starting ether without any response from the idle speed. I also removed and cleaned each electrical connector I could find with contact cleaner.

Before any of this happened the first indication was that I got the EPC light and the related warning symbol and the engine went into limp mode. I pulled over and restarted and the EPC light never returned, but my rough idling symptoms appeared.

As I said above, the engine idles rough only after seeing increasing engine temperature and it doesn't throw any codes until then. Also, if I clear the codes while idling roughly, it will smooth out for a while.

In any case of idle condition the cars runs strong without any hesitation once beyond 1,000 rpm. Especially confusing is the fact that this car (as I said above) only has 46,000 miles although they must have been local and slow trips in Beverly Hills. I've had the car for nine months and it has been awesome until now.

Sorry for this long posting, but I am trying to provide as much background as possible. I can see that this forum topic is very large and wide ranging because of the fact that there are many things that can cause the basic problem and that the codes only identify some of the symptoms. I am hoping the forum can zero in on the full problem by identifying all that I know about this.
Thanks, Paul

 

You probably have a vacuum leak somewhere.
Keep spraying either on a running engine & you won't have to worry about how your car runs any more!

 

I too would suspect the irregular idle to be caused by a vacuum leak somewhere. There is a lot of vacuum hoses, especially on the back side of the engine in between the firewall and the engine where you can't see very well.

I would also begin checking/suspecting the engine speed sensor.

 

Keep in mind you have all the turbo hoses , intercooler ect that are part of the intake system.

 

OK, as a follow-up and in obvious despair I took the car to the dealer for factory diagnosis of the problem. In line with responses to this thread (and other similar threads) the culprit was stated to be the 'spider' large vacuum hose so this was replaced but the result was no improvement in the idle, with codes still indicating lean running on both banks.

The thing is, when warm and idling roughly, clearing the present codes immediately fixes the problem. To me that means that the fault is not a physical problem (such as another vacuum leak) because code erasure cannot correct a real world condition.

Does anyone have a comment or suggestion at this point (please review the initial note in this posting).

I had hoped that dealer diagnostics would quickly pinpoint the problem rather than the poke-and-hope method that I would take.

I will be interested in any responsive comments.

Thank you

 

I would still suspect a vacuum leak as you maybe getting air that the MAF hasn't taken into account, thus it will run lean. When resetting the codes, you take it out of limp mode, so to speak.

Aside from that, I would start looking at the fuel delivery system.

 

Thank you for your reply - I really appreciate it. To help me understand the impact of the fuel delivery issue - is it possible for a low fuel flow at idle, but not and opening throttle?

The vacuum line from the intake manifold to the fuel pressure regulator was one of those that I replaced, but that action had no effect.

My other thought was that the throttle body butterfly was in a position too open for idle mode. But checking with engine off and cleaning in place (a half-hearted attempt I suppose) resulted in no new knowledge or change.

What influence does the Charge Air Pressure sensor have on the system?

Thank you for your continuing interest.

 

To follow up with some closure to my problem, I finally took the car to the dealer. After replacing suspect vacuum hoses, after suspecting the fuel pressure regulator for internal leaking, after removing cam covers to confirm proper cam timing, a provision of a new throttle body has rectified my problem. Perhaps a proper cleaning of the throttle body would have rectified the situation. I would suggest that attempt for others with the same problem because the cost of the dealer services and parts is extortionate.