Intermittent rough idle/vibration

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 :stick:

My first thought would have been a vacuum leak too but when you say no codes then I start thinking elseware. Usually the car will throw a code for a vacuum leak or a lean code. Maybe if it is small enough to affect only at idle and not higher RPM's the computer doesn't pick it up though? I'm running about the same setup as you. 6spd, Catless, Full Miltek downpipes and exhaust, Rebuilt Scroll Turbos, SAI system deleted, Nubcake tune. I have Apikol transmission mounts (Poly) and Stern engine mounts with a 034 Density line front snub mount and I get a slightly increased vibration at idle but it's pretty smooth overall, hardly noticeable. Since you're getting no codes, maybe it's time to do a complete log, either Vagcom 3, 31, 115 I think and see if it all matches up or doing Nubcakes ME7 logger you get a lot more channels but trying to interpret what they all are is difficult.

Thanks for your thoughts...I have some sunny and warmer days coming this week in the 40's, so I'll try to get some time to do a few log runs and compare some of the basics with VAGCOM. I can do the ME7 as well, but as you mention, it needs someone who really knows it to decipher.

Check group 033 with VCDS.
Try to catch the issue with ME7L running.

Slightly unequal MAFs are normal, that's due to the difference in WG actuators.
Suspect intermittent EGT sensor issues. Not consistent enough to trigger CEL.

Other possibility is just exhaust hitting somewhere. Hard to tell without actually seeing the car.
Is the idle really lumpy and/or shakey, or it just appears so with the difference in sound?

Thanks Nubcake...it's been getting a little worse as time goes on, so I'm sure that I can log with the ME7 and shoot you a log.

I'll also check the EGT's and group 033. As far as the exhaust and vibrations, definitely not an exhaust thing, feels more like a balance issue. I can tell at idle because I know the car, most others wouldn't know better...that said, when you rev it a bit past 2200 while sitting still, it gets worse for sure and is a noticeable engine vibration that smooths out some after 2800 or so, but still present.

Long long shots here. One might be the harmonic balancer. The whole drive train is designed around an auto, sub the manual and you have different harmonics.

I get this slightly rough idle very occasionally with no codes. Very minor though and always in neutral but not under load, as in gear in the auto. So many things inc the responsiveness of the DBW system to additional loads like AC and lights etc.

The RPM range replicates a range in my RS4 before the exhaust note changes and it comes on gangbusters. I get similar in the RS6 exhaust and with non factory mounts, maybe that's enough to make it noticeable.

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Originally Posted by IchBautAuto

Long long shots here. One might be the harmonic balancer. The whole drive train is designed around an auto, sub the manual and you have different harmonics.

I get this slightly rough idle very occasionally with no codes. Very minor though and always in neutral but not under load, as in gear in the auto. So many things inc the responsiveness of the DBW system to additional loads like AC and lights etc.

The RPM range replicates a range in my RS4 before the exhaust note changes and it comes on gangbusters. I get similar in the RS6 exhaust and with non factory mounts, maybe that's enough to make it noticeable.

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Thanks for your thoughts here...I figured if it were a balance issue, that it would be either the harmonic balancer or possibly flywheel/clutch package. I may look into a balancer, I did a fluidyne on another car that I had which did seem to work well, not sure if they are available for the RS6 or not, but worth a check.

And Nubcake, I sent you some logs...feel free to tell me that I'm nuts...car does run strong and is a blast to drive!

I use a fluidampr on my 2.7 RS4 with no issues. I believe that I read somewhere that the RS6 version wasn't as successful but I'll need to chase that up to see if I read it right. Good luck with the hunt.

Fluidampr is a great company. I have one on my B8.5 S5, wish they made them for the whole Audi selection. Not cheap though, but a piece of mind.

Unfortunately, it looks like the balancer is only available as OEM, and hard to find/expensive at that. Was hoping that the 2.7 would convert, but apparently not based on everything that I read. Seems that these came on the RS6 and some D8's with the 4.2, so not much call for an aftermarket on these.

I sent Nubcake my ME7 logs for a few different runs, and he didn't see anything obvious...the rough idle isn't hateful, and being on solid mounts I should expect some of this, but I can notice it more at times than others...I may just be too concerned about nothing to fret over....(thanks for the quick replies!)

I haven't driven the car in a few weeks due to weather, so it was a blast to get it out yesterday for some testing in 45 degrees and sunshine...

Did you remove the plug in the crankshaft before you installed the flywheel? Also used thread locker and proper torching sequence on the flywheel bolts?

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Originally Posted by SteveKen

Did you remove the plug in the crankshaft before you installed the flywheel? Also used thread locker and proper torching sequence on the flywheel bolts?

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Thanks Steve fro the thoughts...I did remove the plug, so all good there. Ditto on the flywheel bolts with thread locker and torque sequence. I'm probably worrying a whole lot about a minor issue, but I like to have everything right if I'm going to do the project...the car does run really well and is a blast to drive with the 6 speed, with the help of your kit!

Kicking this back up again...I took the car out last weekend for a longer drive than usual and definitely had a much rougher idle when coming off of the highway to a stop. My usual drives are about 6-8 miles in a loop around town with mostly lower speed (with squirts of power here and there), then a 2 mile or so highway stretch and back towards home on the 30-40 mph stop and go. This time, I stretched out a bit and took it about 15 miles down the highway (plus 5 miles to get there) with some harder bursts, but nothing crazy with too much traffic...once I got to my turnaround point, it was very rough at the stop light compared to normal. Still seems to have plenty of power and no CEL's at all, and settles back to normal with a little more local driving.

Looks like I'll need to run it out a bit to get to the true rough idle point and do some logging...I'd love to be comfortable taking this car on some longer trips, but not there yet for sure.

Not sure if it helps but I had a very similar problem. My WG actuator rod would sometimes rub on something (DP or body?) and get stuck open just a bit. It would eventually go away after it closed only to come back after a hard pull. Tried lots of things and $ to fix it. Once I found it I bent it back and was all good.

Thanks for your thoughts...definitely seems like something quirky like this, I'll take a closer look! Fortunately with the 6 speed, I can actually see them. I somewhat kick myself on these as there were some OEM NOS actuator assemblies that I passed on when rebuilding my turbos.

When I first noticed the problem, it was right after picking up my car from getting the resonators fabricated on the back end...nothing adjusted up front though. I drove about 8-10 miles on the highway this time and first noticed it when coming down to an idle in traffic. Second time was several months later after a similar highway stretch of time...both of which are longer than I usually go at highway speeds...neither runs had very hard pulls as there was traffic both times, but definitely gave it a few punches here and there.

Weather is horrible here this weekend, so hopefully can do some logging in a week or so when I can take it out for an hour up and down the highway and hopefully re-create the issue.