I seem to be experiencing the same problem. After a cold start, i hear a loud sucking noise coming from the rear portion of the engine, and then it disapates and goes away completely after 10 seconds or so. I have noticed a loss in power, and a larger than normal lapse in acceleration time after i push the gas pedal.

Does this sound similar?

I am going to run the CEL when i get home, ill post what the code is when it becomes available

 

Sounds like a vacuum issue to me. Check your intake and airbox. You may also have a leak in the lines somewhere.

 

Does it 'spit' the stuff out? If so, most likely a blown head gasket.

Oh, and O2's do effect mixture, but if they go bad or are removed, the ECU defaults to a pre-loaded fuel map. Sucks for mileage.

 

Doesn't spit it out, just noticed a bit of pooling. It may be just a LOT of carbon mixed with water.

 

Ya.. could be.

 

Sounds like a blown head gasket to me as well. I would get a compression test done immediately to check for carbon in the coolant. I would also have them pull the plugs and check for moisture. If you haven't had any overheating yet you may be in a good position to get it fixed, but it will cost a pretty penny. If you catch it early , it could just be a small amount of coolant getting into one of the cylinders so it will make it run rough primarily at startup. Lets hope you don't get backwash of your coolant and overheating that can cause real catastrophic problems. I hope I am wrong here for your sake. My Range Rover passed three compression tests before I finally overheated after replacing multiple other things. Good luck to you. Maybe it is something small. If it overheats it will be immediate and DO NOT DRIVE IT HOT!!! A tow is cheaper than a new engine. Also, watch your coolant level.

 

UPDATE:
Took it to the only gig in town that could read an Audi's codes. They test drove it and thought she ran fine. They did find the old P0421 code along with a Diagonal Position Sensor error code. This morning, I had the traction light come on and stay on while the engine shook under acceleration. Idle still is high and rumbly on cold starts.
They're going to replace the spark plugs and have the fuel injectors cleaned properly. Thought I'd get some input. Thanks!

 

UPDATE: Well, it's a good thing I had that done! The injectors were HEAVILY gunked up. They also discovered that when my air pump was replaced, two of four vacuum lines were cracked. Sounds like I've found my power-sapping problem. I thank you all for your help and advice!

 

how much did the injectors cleaning job cost?
tks

 

change the cam shaft position sensors, i had the problem with my car and it fixed it.. btw, this is the third thread i have said that in today.. they seem to mess stuff up bad, so dont overlook them!! quick and easy to change and only 200bucks!