GOLFMK8
GOLFMK7
GOLFMK6
GOLFMKV

2012 GTI misfiring at idle. Out of ideas

AntiGTI

Passed Driver's Ed
Car has been misfiring at idle. Had P0300 and P0302. Changed spark plugs, checked the gap, and changed the Coils since it had been a couple years. No change in misfires. moved coils and plugs around with no change.
Took off the IM and removed the carbon build up (was esp bad in cyl. 2) and replaced all 4 injectors with new ones.
Still had P0302. took IM back off and replaced the IM gasket, and double checked all my work.
P0302 gone (yay) but now I have P0303 WTF.
Replaced the PCV valve, and it had def failed, but the P0300 and P0303 remain at idle. Car feels great when driving, but flashing cel and constant misses in cyl 3 at idle.
Compression tested 155 on all cylinders.
I sprayed carb cleaner on all the vacuum hoses and around the manifold, but didn't find any changes in idle.
Cleaned the MAF with no changes.
Removed the TB and cleaned it up, with no change.
I have OBDeleven, but not sure what to look for besides watching the misfires count like crazy when I'm stopped.

Any ideas of what to do next? I'm about to admit defeat and bring it to a dealership. Seems like it could be something simple.
 

supraeazy

Passed Driver's Ed
If its a constant miss on just 1 cylinder and you've performed all the tests that you listed then I would either try a leakdown test, which I dont personally think would uncover anything, or you need to check circuit integrity of your #3 ignition coil and injector. If you dont have access to wiring schematics or repair info I would be able to provide some so just let me know.
 

AntiGTI

Passed Driver's Ed
I don't have a leakdown tester, just compression, but I can I can check the wiring. I did suspect that as a possible problem, but was dumfounded as to why it would have changed cylinders from 2 to 3 after redoing the IM install.

I do have a couple broken coil connectors. Replacements showing up in 2 days. They are on cylinder 1 and 2.
 

king1138

New member
Do you have the newest revision intake manifold? It's possible the flapper is damaged and causing air to flow incorrectly at idle. Mine would misfire like crazy at idle until I had the new intake manifold done.
 

AntiGTI

Passed Driver's Ed
I did have the IM replaced under warranty at around 75k mi. This was in 2017. Not sure which revision was used at the time though. When I had the IM off recently I articulated the flapper and cleaned the flaps off. It seemed to function as expected. I have tried to spray carb cleaner around the arm that moves the flaps and along the IM tubes for leaks.
At this point I still don't know that I can rule it out as a cause but I haven't seen a P2015 since then. I'm up for any test, or value checks in obdeleven to help rule it out.
 

AntiGTI

Passed Driver's Ed
I ended up bring it into a shop, and they did a compression / leakdown and everything was good. They borescoped it and found excessive oil in cyl 3. They believe the rings are failing in that cylinder causing the misfires. They would have to remove the cyl head to inspect further, but likely think it's going to be a rebuild.
Before I RIP this car, does anyone have any last minute YOLOs I can try? Otherwise I'm gonna drive to a dealer and trade it in as is.
 

AntiGTI

Passed Driver's Ed
Thought I'd update this. Feeling kind of skeptical of the garage's diagnosis since the compression / leak down was all good. I decided to try one last time to swap the injector and see if the problem follows it. Welp it did. moved the #3 injector to the #1 spot and now I'm only getting misfires in cyl 1.

So looks like against all odds, one of the 4 injectors I got was bad and I happened to install it in exactly the same cylinder that was misfiring to start with.
Ordering another injector and hoping this could actually fix the problem.
 

supraeazy

Passed Driver's Ed
Thought I'd update this. Feeling kind of skeptical of the garage's diagnosis since the compression / leak down was all good. I decided to try one last time to swap the injector and see if the problem follows it. Welp it did. moved the #3 injector to the #1 spot and now I'm only getting misfires in cyl 1.

So looks like against all odds, one of the 4 injectors I got was bad and I happened to install it in exactly the same cylinder that was misfiring to start with.
Ordering another injector and hoping this could actually fix the problem.
Right on, yeah the excess oil in the cylinder seemed like a weird diagnosis especially since a wet compression test could have easily confirmed worn rings...

I would possibly find another shop in future.

But glad you got it figured out and seem to be on the path to getting it fixed!
 
Top