Three problems related to engine have been reported for the 1999 Audi A6. The most recently reported issues are listed below.
Unintended acceleration -- car accelerated rapidly. Would not slow even when driver's foot was not on the gas. Engine revved rapidly and would not return to normal idle speed.
I had a leak in the cam seal and it seems that a lot of others have experienced it in the Audi forum.
Cam seals and main seal both failed and cost over $2000 to repair out of warranty. Very upset since the car only had 58000 miles on it.