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.

Symptom; 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 that’s 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, It’s for torque anyway I wasn’t 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 I’m 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.

 

How did you clean the injectors?

I still have the fuel rail entirely out (injectors still connected on it) and although the car was "normally" responsive on acceleration and had a steady idle, would like to attempt to clean them.
There are no dealers/shops around here that clean injectors other than IN THE CAR (they do some sort of treatment by disconnecting the fuel feed, connecting the rail to their system and run the engine). Probably Seafoam or that German "Ventil Sauber" (by Lubro-Moly) would accomplish the same thing 20 times cheaper.
Anyway, I have new 0-rings - although I had no fuel leaks, and I would like to try to clean the injectors before reassembling the heads.

Thanks (for the "fixes" list as well).

 

As you already know if they were working fine they are probably not dirty.

I took a Magnifying glass and visually inspected each tip to see if I could see anything.

First I removed the O-Rings from the injectors, I put a small amount of sea foam in a cup and soaked just the tips for about ten minutes. Tap them to get the air out so the sea foam gets into the injector.

Then I took a brass wire brush and carefully cleaned the carbon from the sides.

I rigged up a 9 volt battery with two alligator clips to actuate the injector.

I turned my compressor down to 20 PSI, and put the air gun in the vise. and had a spring finger clamp to hold the trigger so when I was ready it would deal with itself. If you don't have a compressor you can get cans of compressed air for computers and electronics anywhere.

I actuated the injector and poured some sea foam into it. brought it to the air and blew it out back wards quickly so I did not actuate them too long. I went down the line and did all six. Then went back and blew them out the other way. You can see them spray, and watch the pattern if one only mists out one side you would see it.

I cleaned the terminal pins on each injector with a nail file.
(You would probably need some mission impossible music to get past your bride who is no doubt still Pissed about the whole valve covers in the oven incident- How are you healing by the way?) I scuffed the tops and bottoms of the pins. When the file gets dull clip it down with a pair of dikes.

I replaced all the o-rings (although the old ones looked OK) they were .92 cents each-$12. after you put them on the injectors lube them before you insert them so they do not tear. I use Vaseline (but just in case you are still on the Lamb with the nail file) engine oil is fine. BMW and Land Rover use the same size o-rings if that helps any.

Put some DI-Electric grease on every terminal before you put it back together. I use it religiously I'm originally from NY and the salt is attracted to the static. A friend of mine says a $5 tube makes half the difference on a BMW tune-up.I just slide it across the 2 metal females in the plastic terminal and fill them that's enough.

 

Truly great insight (and inventivity!).
I appreciate very much your thorough explanation and the time dedicated to do it.
I got my o-rings from AAz on the last shippment (83 cents/piece - $ 9.96 for all 12). I'll definitely try to clean'em up; at least just for the fun of it.

THANKS MUCH, sir.