I haven't found any good threads on this so I wrote this up.
Hope this helps those of you with an annoying Vibration.
I bought my 02 A6 3.0 Quattro Avant with 113K. It had a vibration on idle that you could feel through the steering wheel and brake pedal. I left it for last because I intended to completely service the car and figured some of the issues would disappear along the way. There were several things contributing to this Vibration which made it difficult to diagnose and get rid of.
Car Vibrates on idle in D with A/C on, vibrates enough to be uncomfortable on your leg while holding the brake. It was apparent in N with A/C off, but 80% worse under the load of the transmission and A/C. No Codes. Vibration stops on higher RPM.
To start all fluids were changed and filters were new. No oil leaks at all.
Here is a list of things that were causing the vibration.
I cleaned and applied di-electric grease to every terminal I could find.
I noticed a 15-20% improvement when I cleaned and greased the injector and coil terminals. I used a nail file to rub the pins and then applied the grease to the female terminal.
I noticed a 20% improvement when I replaced the coils, I also replaced the plugs but they did not improve the Idle Vibe. I put a screwdriver tip full of Di-electric grease on the top of each spark plug before I installed the coils.
I noticed a 20% improvement when I replaced all of the vacuum lines with new OEM.
I went with the braided from http://www.ecstuning.com/
. They lasted 100K the first thatís fine for me.
I noticed a 25% improvement when I replaced both Engine mounts, the hydraulic fluid had leaked out.
The snub mount that was completely disintegrated made no improvement on idle, Itís for torque anyway I wasnít expecting much.
I tuned out the rest by adapting the idle with the Vag-Com and adjusting the idle to the smoothest rpm range.
NOW WHEN I START IT WITH A/C ON I CAN NOT FEEL ANYTHING.
IN DRIVE STOPPED AT IDLE I FEEL IT RUNNING, BUT UP TO STANDARD VERY ACCEPTABLE. Same as any other car.
Here is a list of things that made no difference to idle.
Keep in mind some of these items were not bad if they had been Iím sure it would have been a different story.
It ran smoother over all when engine oil was changed but it did not affect the idle.
The Sea foam treatment had no effect on the idle vibration but had a 40% effect on startup noise and vibration.
Cleaning, testing and replacing o-rings on the injectors-They were working fine to begin with.
Cleaning the MAF, was easy but did not make a difference.
Spark plugs, they were all in good condition, NGK
Transmission service, was a PITA and did nothing for the idle.