Lumpy idle on an '08 GTI

[avl]

Hi everyone,

Looking for some advice regarding a lumpy idle on a 2008 spec GTI.

I bought the car with 106,000 kays on it, and the idle was dead smooth. Within a month or so it started to get a little lumpy every now and then.

It's become progressively worse ever since.

It's at its worst when the car is up to temperature. It's quite bad if you have the aircon off, and a lot better with the aircon on – although you can still feel it in the seat and steering wheel.

The car still pulls great and there's no sign of a misfire or anything once it's got some revs and load on.

The first thing I did was replace the PCV just in case, which made no difference, and I even threw top engine mounts at it as one mechanic suggested that was the issue. Made no difference either.

It doesn't throw a single code, and it's just had new spark plugs fitted as part of the 120,000-kay service.

I'm thinking the culprit is build-up on the intake valves.

Is there anything else that might be causing it? Is it worth biting the bullet and ripping the intake manifold off to clean the valves?

I've had a couple of mechanics look at it and the reaction is generally 'ah, it's not that bad', but it drives me absolutely nuts whenever I'm sitting at a set of lights. I'd love to get it sorted so I can just enjoy the car.

Any advice welcome.

Cheers,
AVL

[Lucas_R]

With those km's and age of the car, it could most certainly be carbon deposits on the injectors and valves due to the direct injection - these engines are notorious for it.

Another thing to check/replace would be the ignition coils - although you said there is no misfire under load so this may not be the issue right now. New coils can be purchased for under $200 for the full set. Audi R8 Ignition Coil Pack Set of 4 - 06E905115E - MK5 GTI (2006-2009) - GTI/GTD - Volkswagen - Vehicles - SHOP BY

For the sake of $20-25 i would go to Supercheap Auto and buy a can of the Liqui-Moly intake cleaner spray. On the Mk5 GTI, there is an air temp sensor on the front of the inlet manifold which is perfect to remove and spray in the cleaner product. Liqui-Moly Petrol Engine Intake Decarb - 366g | Supercheap Auto

I have used this previously on a Mk5 GTI (mates car), and my own Mk6 R and Mk6 GTI as preventative maintenance.

[avl]

Thanks Lucas. I've wondered about coils but the fact it runs so well under load has always put me off that being the issue.

Can those cleaners make much headway when there's already significant build-up there? Or is it a case of needing to rip it to bits and scrape that junk off?

[h100vw]

I'd plan to remove the manifold and get physical. All the ones I have done wouldn't have been improve by any spray.

[Shadow_Rusty]

I had a lumpy idle on my Eos shortly after getting it.
Turned out to be the PCV Valve assembly.

Try removing the oil cap whilst it's idling, if it doesn't change the way it idles, it's probably the PCV that's failed.