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1999 Audi A6 Avant occasionally stumbles, throws CEL

We have a 1999 Audi A6 Avant (wagon) with about 190,000 miles. Best car we’ve ever owned, and want to keep her running until the wheels fall off (so to speak). We split our time between sea level and a ski town at 7000’ (yes, that may be relevant).

Last spring our Trusty (sea-level) Mechanic repaired the exhaust system; both tailpipes were shot (car sounded like a tank). Rather than replace with Audi parts (was going to be a full exhaust system replacement - $000s) they spliced in some new tailpipe. Car ran quietly after that.

However, immediately thereafter (like, on my drive home from Trusty Mechanic’s shop) and ever since the car will occasionally stumble (misfire) and throw a check-engine light. Typical scenario is after having been off the throttle while going downhill and then tipping into the throttle at the bottom of the hill, the car will stumble and on comes the CEL. Stumbing lasts 5-10 seconds then gradually subsides. Light stays on, though. Not 100% reproduceable under those circumstances (but doesn’t really happen under other scenarios), but happens often enough to be disconcerting.

We recently decamped from sea level to spend the summer up in the mountains (great bicycling), and while the car misbehaved a couple of times on the way out of town, by the time we got to 5000 ft or so, the CEL was no longer illuminated. The car has not misfired once while at altitude despite plenty of situations that would presumably have triggered the issue, were we at sea level.

Trusty Mechanic has replaced some injectors, some coil packs, and the spark plug cables. No behavior change.

My wife and I suspect that something got bollixed up when Trusty Mechanic was working on the exhaust; perhaps an OXS-related problem. He doesn’t agree.


So has there been any kind of scan done on the car or were the changed parts more of a guess at it than anything else?

At 190k miles there’s a lot of possibilities but knowing why the CEL is illuminated has to be the starting point for any guessing here. :slight_smile:

+1 to OK4450’s post.

I also find myself wondering if there are any other symptoms such as excessive oil use, puffs of smoke upon startup in the morning or at the bottom of the hill when you again press the gas, black smoke when you press the gas, or anything else… even things that might seem unrelated?

Has anyone looked at the spark plugs lately? How’d they look?

The new tailpipe, how did they splice it? Did they weld it? Did they weld it to the rear cat converter (if you have one)?

See if there are some stored codes and report back. Codes will be in the format of P0320.

Since is asking for haikus, here’s mine:

Trusty mechanic
Goofed up the exhaust system
Sad Audi owner

With out the actual code we can you, yep somethings wrong. But we can not definitively say what or that the exhaust work caused anything or not. Get the codes read and c’mon back.

Okay; will ask TM for the codes and post them here (I’ve not yet received my ScanTool). He has typically related that the codes he sees relate to misfires in various cylinders, but I don’t know the exact code values. The parts replacement(s) that he did were spurred by his interpretations of the codes (as opposed to randomly replacing parts and hoping for a change in symptoms, though after awhile it kinda feels like that, no?).

Old consumption, smoke, etc.: nothing like that. Car runs great the rest of the time and does not have increased oil consumption.

According to my records, plugs last replaced in 2011 at about 150,000 miles. TM has not yet billed for the current series of procedures, and I can’t recall whether or not he has replaced the plugs as part of his efforts of late. I will check with him on that.

As for the exhaust system, the mechanics welded in new “flex hoses” on both tailpipes (dual exhausts). I’m not familiar with where those flex hoses would connect (sorry).

Hope that’s somewhat helpful.

OK, I asked TM for the codes. He responded as follows (yes, I have asked him for the actual codes again):

So if the car is running ok at altitude i suspect lean misfire at sea level, possible air mass sensor correction fault. parts that have been changed are coil pack, plugs and plug wire set, fuel filter and number 5 fuel injector. your last time in you has fault code only for number 5 so i installed a new injector on 5, we also smoke tested for unmetered air leaks and found air horn to throttle body gasket was leaking and installed new gasket. your past misfires were on cylinders 2+4 at one time and 4+1 at another. lean trims looked good and oxygen sensors were in range and switching rich/lean correct. I was never able to duplicate any misfire except when we changed the coil pack for poor running

TM reported back regarding the codes. He reports that he’s only seen misfire codes: “misfire codes always start with PO and then the number. So po300 is random misfire, po301 is cyl 1 and so on, cyl 6 would be po306”

If the issue is, indeed, “lean misfire at sea level, possible air mass sensor correction fault”, then (1) what are likely causes and (2) what are likely fixes? Would that result in (only) misfire codes?

Have you cleaned or replaced the MAF?

Unplug the MAF sensor and go drive it. If the problem goes away then the MAF is likely bad or there is an air leak in the intake tract somewhere.

Latest update. Car continued to run fine at altitude until 15 October, when there was a major “event” (at the beginning of a planned 800-mile trip!). Serious stumbling, much more severe than previously. CEL on (flashing at first; solid afterward). Eventually, car smoothed out and ran OK. Finished the trip; by the end, the CEL had gone out. TM diagnosed coil-pack failure - replaced and that serious stumbling went away.

Now we are back to the original symptom: occasional stumbling (misfire?) with CEL upon tipping back into the throttle after being off-throttle for a bit (say, at bottom of a hill, coming out of a clover-leaf, etc.). TM can reproduce - yay! His diagnostic readouts show the car going completely lean even though the throttle is open. He has replaced one “marginal” O2 sensor and cleaned the MAF - no change in behavior (though readouts “look better” to him afterward).

Recall that all of this started after TM’s staff had replaced the flex lines, both sides, forward of the catalytic converters.

(I apologize for the time-span between posts here. Not evidence of unconcern on our part. We don’t drive that much, but when we do it’s longer distances, mostly. So time passes between events. Thanks again, all, for your help.)