Unsolved Issue, Slight Rough Idle - no CEL

After searching dozens of threads I can't seem to find any with the exact symptoms I'm having - looking to the forum for any input.

2013 GTI, 6sp, completely stock drivetrain other than APR stage 1 CAI, APR Catch Can and R8 coils. 85k miles.

I find I have a rough idle 70% of the time at a stop. Not enough to throw a CEL, but enough to be noticeable. slightly rough. This happens when cold, warm, on a short trip, long trips, driving easy, after driving hard, no matter. May be random but I find it is more prevalent on short trips.

Only happens at idle - at speed it's perfect. Pulls fine, no dips in power. Fuel economy good.

Items to note:
R8 coils and plugs at 56k miles
Carbon cleaning (properly, intake off, media blast) at 83k
At the same time, injectors completely rebuilt at 83k
New air filter & OEM fuel filter at 83k.
Always Shell 91 V-Power
Always Motul oil every 5k miles
New-style timing tensioner
Latest rev diverter valve (rev D?) at 83k. Not sure if this would matter here.

I was thinking the above work would fix the problem (and it is better) but still there.

I had the timing checked and I'm still within spec at -2.6
Also scanned the ECU before the above work and it noted random misfires, but again not enough to throw a CEL. Haven't had it scanned after the work.

Maybe intake manifold? Maybe cam position sensor? PCV?

Have you checked your mounts? Engine side, trans side, and dogbone mounts? Any one of these could be your issue, since it sounds like your engine is pretty healthy.

When you look at the engine is it running rough or just vibrating? A vacuum leak will cause a rough idle. If it's only 70% of the time what is going on the 30% when it is running smooth?

torga said:

Have you checked your mounts? Engine side, trans side, and dogbone mounts? Any one of these could be your issue, since it sounds like your engine is pretty healthy.

Click to expand...

It's a good thought - I'll take a look. The fact that the ECU had some misfires had me thinking something else, but I'll check and re-scan.

Misfires too, huh? In that case, do you know if your intake manifold is the most recent revision? Have you seen the P2015 code when you've looked?
Sometimes when the old manifold fails, it doesn't hard code a CEL, but you'll see the P2015 code if you scan. Same with a P0300 and a few others corresponding to each cyl that sees a misfire.

Yeah this is why i preach new intake manifold when u carbon clean literally same labor just pay for parts.

As for mounts I've noticed those are more clunks rather than rough idle but a definite possibility.

I also do plugs every 40k but ur still shy of 10k for that.

Double check for vacuum leaks.

Sent from my SM-G950U using Tapatalk

I had this same problem i waited till i got a cel which was p2015 intake manifold unfortunately my vin wasn't covered under extended service campaign. So i paid oit of pocket anyway got carbon cleaning while they were in there and it went away.

Sent from my SM-G950U using Tapatalk

torga said:

Misfires too, huh? In that case, do you know if your intake manifold is the most recent revision? Have you seen the P2015 code when you've looked?
Sometimes when the old manifold fails, it doesn't hard code a CEL, but you'll see the P2015 code if you scan. Same with a P0300 and a few others corresponding to each cyl that sees a misfire.

Click to expand...

Appreciate the input - I'll check the codes again and see.

Boscogn said:

Yeah this is why i preach new intake manifold when u carbon clean literally same labor just pay for parts.

As for mounts I've noticed those are more clunks rather than rough idle but a definite possibility.

I also do plugs every 40k but ur still shy of 10k for that.

Double check for vacuum leaks.

Sent from my SM-G950U using Tapatalk

Click to expand...

The classic VW "well while you're in there, might as well replace..."
I think you're on the right track though. I'll check both.

To update, if anyone else has this problem - replaced the PCV with the latest "AH" revision. Didn't think this was going to help but I was going to replace this anyway to avoid future issues and it was cheaper than an intake manifold. No difference. Going to keep watching the codes and do the intake when the time comes.

My guess is carbon buildup/intake manifold.

Update here - car was stored for winter.

Put the latest revision intake manifold on at 94k miles. Mechanic did a quick media blast touchup on the carbon as well since he was in there (for free!). Slight rough idle is still there... going to do a check for vacuum leaks, otherwise I'm going to put this into the "it just does that" column.

Also to add - I did new plugs in Sept 2019 @ 91k miles as well as fuel injector cleaner. No change.

Why dont you pull logs during idle?

when you feel the rough idle does your speedometer needle bounce a tiny bit? if so vaccum leak. if not, like mentioned above, could be worn engine mounts since it happens at alot. Have you replaced your PCV lately?