Having a few difficulties with my college student daughters A6. Amongst other issues, the main thing right now is that it idles like crap and sounds like it is sucking vacuum from the crank case vent. Bought a new CC vent, but same issue, maybe worse.
But when it warms up (even in the summer heat) it purrs like a kitten with not a problem. Yes it has the ever present CEL on, but thats normal!
Any suggestions before I keep chasing parts with good money?
Signed,
A father of a college student who would much rather throw my money at her education than her car! Thanks!

 

The first thing on my list would be to find out why the CEL is on. You mentioned possible cats in the previous mileage thread. Are they bad and do you have testing? I used aftermarkets on my 02 4.2. They both started rattling after 6 mo and not many miles. Replaced them under warranty but had to pay for labor. After the last one started to rattle, I discovered it is downstream from the sensor and let it rattle apart.

The OEM are about $1,000 each for a car that is worth maybe $4,000. I like the way my car drives but would never do this for a college car. My daughter has a Civic that generally doesn't cause trouble.

 

I had bad cats in my daughter's 2003 A4 3.0 (replaced by dealer under warranty at 79k) and my 2.7T (replaced with Billy Boats at 180k). Neither caused a rough idle. Both threw CELs. So pull the codes or have someone like Autozone do it for free. Sounds like a vacuum line problem if it is sucking air. (I have read) If you get a can of starting fluid and spray it around the hoses, the one that is sucking air should pull it in and the engine will accelerate.


I am puzzled that the bad idle disappears when it is warmed up. Generally a vacuum leak does not go away.

 

Thanks for the replies! I followed what another poster on the forum did and put in some fresh high octane, put it sport mode and rev her up and gave it a good beating. FWIW, It seems to have worked, At least for a week now. Maybe there was a clogged injector or possibly a small pc of carbon holding up a valve? I'm not completely sold that it was that simple of a fix, but no complaints so far .

 

I plan to take it back to my friends garage and have the codes checked again, do a full fuel service and reset the codes and see what new ones show up.
I am leaning against it being anything with the cats because when I took it up the mountain in my hell run with it, I had zero catalytic converter smell or rattling noise from them and it ran awesome and still does. Really not much of a mechanic, so i am still skeptical on the whole situation. Crossing my fingers! I guess I'll see when my daughter comes home and actually WANTS to talk to dad! That's usually a bad sign, HA!
Thanks again and ANY more info is completely welcome as who knows if this will return or not!

 

UPDATE!!! Bad idle returned with a vengeance a couple months later! when she returned home for Christmas, there were a TON of codes! Was told there is no possible way that all the things were bad and something was throwing everything else off. My buddy narrowed it down to what he thought and we put in a new MAF ssensor and the idle immediately smoothed out and only a few codes.
Earlier I said the crank case sensor was whistling, but when we put on 2 different new ones, they were even worse, so I put the original one back in back then. Well, now I put in the new CC vent after the MAF change and all but 1 code went away and she is running smooth now for 4 months- even through a brutally cold PA winter this year.
Thanks to all who responded and pm'ed me on this. Hope my experience can help someone else before unnecessarily dropping a few grand on cats!

 

I hope it's not cats. I had no issues with them but had brought the car in because of an ignition coil recall. They were good, but they found the cats were "bad" and -- guess what -- the warranty on them was done at 80k miles and the car had 79100. I think it was a money-making ploy but it did not cost me anything. A4 ran fine before and after the cats were replaced.