Hey all,
I've been trying to figure out an intermittent rough idle on my RS6. As most of you probably know, I did the 6 speed transmission swap and finally finished it up this past October. The car runs strong and I've ironed out most of the problems. I have a few more things to work through like the Airbag light being on now after sitting for so long and other little nagging issues that can be for another day.

Here's my issue: The car has been running strong and Nubcake has been great to fine tune things for me. That said, I noticed after driving the car for a few hundred miles and on the same trip after getting my resonators installed, I was driving the car on the highway for about 8 miles and not pushing it hard, but a few squirts of boost here and there. That said, there was stop and go traffic on the road, and when I came down to an idle, I immediately noticed that the idle was rougher than it ever has been...not horrible, but definitely noticeable. The car still had decent power from what I could tell, but with traffic, hard to know for sure. On that same trip, I stopped off for an errand (beer run!), and when I got back in and re-started the car after being off for 5-10 minutes, it was back to it's original butter smooth idle again. Now, for the last several weeks, when I do have the chance to drive it between snow, it seems like the rough idle is more consistent and not coming about only after being boosted a bit while driving.

Diagnostically, I reached out to Nubcake, and we agreed that this all started soon after I got the final tune situated, so maybe I was running on the rich side for some of the hard log runs earlier on and maybe fouled the plugs a bit with earlier tunes. Mind you, no codes whatsoever either then or now for misfire, etc. CEL has never come on and the scan shows clear for anything stored. I did go ahead and put in new plugs, which didn't really make any difference as I suspected after pulling the first set out that was still brand new after the rebuild...plugs looked perfectly fine and even. I do have the Accel yellow coil packs which replaced my old mis-matched set, but again, no codes for any misfires.

A few notes:
SAI is deleted and coded out, no cats in the exhaust.
Never a CEL or code that has shown up.
I did a compression check, and all cylinders were excellent and within 3-5% variance.
I noticed that one MAF was reading lower than the other at idle, but under slight load, they basically balance out, but the passenger bank always reads slightly lower.
I did do a smoke test to check on any leaks from the N75 when I thought the MAF readings were off, no issues.
I did replace the check valve up front with the 034 billet version, my original was not holding as it should, but still no fix (also did suction jet pump about 2 years ago).
I did replace my diverters with the rebuild with APR R1's, I checked these and they seem to function properly.
I did check the cam chain timing/phase and they are within spec, just double checking after the new belt, chains, and tensioner shoes.
I do have solid mounts, so I expected some additional vibration, but like I mentioned, this was intermittent and now most of the time.
I do notice a heavier vibration when revving up between 2200-2800 rpms and smooths out a bit over 3k while car is not moving (no driveline issues to consider).
I never turn on the radio and usually open the windows even when freezing out to be in tune with the sound and feel of the car, I'm quite in tune with it and why I immediately noticed the rough idle/vibration when it started.

So I'm somewhat at a loss...I was hoping for maybe a vacuum leak somewhere, but I can't seem to locate one. That said, would a vacuum leak cause the engine to have a vibration when revving without a load? Hoping that there is not a balance problem with the flywheel/clutch setup. Oh yeah, same vibration with clutch pedal out in neutral or depressed.

Any thoughts or specific running values to check on would be welcome!

Thanks in advance, Eric