All Things Mk5 > How to Guides / Troubleshooting

Carbon on valves?

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Chickensaur:
I made a post a while ago about my GTI having 2 knock sensor codes. Lately that has developed into a full-on misfire, but only under hard acceleration (so far just in first above ~5k). I replaced the coils and plugs, which helped a lot but it'll still misfire, just doesn't lose power as quickly as before. Anyway after some research I learned a really common issue that causes this is carbon buildup on the valves, which sounds like it could be it. The car has almost 100k miles, I think it's like 300 away. The misfire is always in cyl 2, wondering if theres anything else to look at.

mjmallia:
Carbon build on the valves is very common with direct injection.

Clogged injectors is also another

breeze:
I thought injector as well. If you are close to R-Tech give them a buzz. They do a reasonably priced inlet clean and injector service, and will have new injectors if you need them.

You say you replaced coils and plugs but you may want to swap the coils at least to see if your misfire moves. Very unlikely but zero cost. You could also try a couple of tanks with injector cleaner in it. I’ve used Liqui Moly 5153 before. No harm in trying.

mjmallia:
At those miles I would say you def need a carbon clean for sure if it has never been done.

SANDEGTI:

--- Quote from: breeze on October 03, 2022, 09:55:43 am ---I thought injector as well. If you are close to R-Tech give them a buzz. They do a reasonably priced inlet clean and injector service, and will have new injectors if you need them.

You say you replaced coils and plugs but you may want to swap the coils at least to see if your misfire moves. Very unlikely but zero cost. You could also try a couple of tanks with injector cleaner in it. I’ve used Liqui Moly 5153 before. No harm in trying.

--- End quote ---
Do you put liqui molly in just after you filled the tank or does it not matter and can put it in any time? I ordered some last night, have a rough idle and with 140k I think they are the original injectors. Hoping this improves it somewhat.  :smiley:

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