2005 A6 4.2 SLine that has me stumped. I get a rough idle after driving for a while. Short drives are okay and drives at night or during cooler temps seem better. But I have been having idle issues when its hotter out after longer highway drives over 80 miles, when i come to astop i get the rough idle. After searching forums and trouble shooting and talking to the dealers things seem better but not gone... initial engine diagnostics after i caught the check engine light on was multiple cylinder misfires, coolent temp sensor malfinction, motor mount malfunction...
Here is what I have done so far...
Replaced 8x Ignition Coils
Replaced 8x OEM Bosch Spark Plugs
Replaced MAS and air filter
Replaced Coolent Temp Sensor
Replaced 1x Dynamic Motor mount
after all this things seemed fine but first hot day long drive i got the rough idle. if I shut the car off it may reset temporarily but seems to come back depending on how much heat sync has happened is all i can figure.... The check engine will fluctuate and sometimes stay on for multiple days then seems to clear itself? any help would be appreciated!

 

The motor mount issue should not affect the running of the engine. Get the OBD Codes and post them when you can.

 

I just did the motor mount cause it was the only persistent code... I hate having fault codes... Just had it checked again and only thing showing were random misfires and no associated codes... Only thing I hate worse than codes is problems with no associated codes. I noticed that once the check engine light comes on and stays on the misfires seem to remedy themselves.

 

Definitely need to get the code(s) before the check engine light turns off.

 

The shop I stopped at to read the fault with the check engine light on only pulled the misfires with no other codes. They told me if I came back they can dissect the fault like a snapshots prior to the check engine... See if they can see any engine anomalies... Is this right? They said if the check engine comes on the computer records the fault even if it goes off...

 

Yes, the computers stores ALL of the codes until you clear them even if they've corrected themselves and the CEL/MIL light goes off.

One suggestion, locate the grounding points on the wiring harness on the engine and the ground straps from the engine to the body of the car. Loosen and retighten them reestablish a good connection or even take them completely loose and clean any corrosion you may see and reattach them (use some anti-seize compound on any bolts that may go into aluminum parts). Lots and lots of crazy electrical issues are simply the result of a corroded ground connection.

 

Lordimp
did you ever fix the problem?

I'm having the same issue with my 2005 A6 quattro sline 4.2