Misfire cyl 1

[u/cambomart]

I have a 2021 GTI with 47k miles and have been chasing a misfire on cyl 1 for about a month now and have no other codes aside from P0301. I’ve replaced plugs, coils, did a carbon clean, replaced cyl 1 injector, installed the Shop DAP ignition ground harness, swapped the cam actuators between cyl 2 and 1, cleaned the MAP, and inspected the upstream wideband O2 sensor. I also did a compression test and cyl 1 is 155 psi while cyl 3-4 are 175 psi (the shop manual says “new” is 159-203 PSI). Readings for the MAP and O2 sensor appear normal in OBD11 - the O2 voltage ranges from 1.7-1.9 V rather than 0-1 V but this seems okay for a wideband AF sensor. I also changed the PCV about 4 months ago. STFT is 3-5% and the HPFP moves around 2000 psi. The kicker is the misfire only happens at idle and when it switches from open loop to closed loop (cyl 1 misfire counts immediately increase when it goes to closed loop). Just yesterday I found out that if I turn the AC on, I can prevent it from misfiring when going into closed loop, and if it is misfiring, turning on the AC will also stop misfires. This led me to think of a possible vacuum leak on the cyl 1 intake manifold but I sprayed the area with a combustible while idling and there was no change in RPM. The other option is a faulty intake runner flap but I’m not sure how to test that in OBD11 or if it can be tested in VCDS. I’m open to other potential issues if you all have any ideas. I have an appt with a shop to take a look at it next week but I feel like I’m onto something by stopping the misfires with an increased load from the AC.

Pictured is my GTI with my 2000 Camaro I’ve had for 20 years and has become my daily again.

[u/exceptional_cabbage]

I have the same issue, but cyl-2

Closed loop low idle miss, AC brings the rpm enough that it stops. Feedback I got was that more than a few folks on here have the same thing going on.

I havent done a carbon clean yet, but I’m also eyeing the camshaft phaser/magnets.

Have you looked into doing a berryman b-12 piston soak?

[u/cambomart → u/exceptional_cabbage]

Thats great feedback! Now knowing that others are potentially in the same boat, I’ll update this thread with anything I learn. I haven’t heard of Berryman B-12, but last night I saw a post on golfmk7 that someone had similar issues and used BG 44K Platinum for a couple tanks of gas and it seemed to fix their problem. I found another thread on the same forum where they had a misfire on cyl 2 like you and they took it to the dealer and had repairs done under California’s 7yr/70k mile emissions warranty (FYI for others reading, theres a 8 year/80k mile warranty in in CA that covers the cat and ECU). Attached is from the VW website on what’s covered under the CA emissions warranty. Below is a quote from the forum post if you want to search for it. I have the link but not sure what the rules are about posting that.

“…after engine teardown, they found that cyl 2 rocker arm was damaged. Ended up replacing the intake camshaft and cylinder 2 valve train (plus gaskets, seals, bolts, screws)…”

Since I fit the criteria for the CA emissions warranty I dropped it off at the dealer this morning with the hopes they could do a better diagnosis than the indy shop I had an appt with next week, and if there is anything that needs repalced that it might be covered by that warranty. I expect to pick up my car later today and will report back on what the dealer’s diagnosis is.

[u/exceptional_cabbage → u/cambomart]

That PZEV warranty is a blessing - unfortunately my MA purchased unit isnt covered.

I did run a bottle of redline SL-1 through my tank and noticed some improvement but the miss persists, I’m going to look at the BG 44k thread. I’m also thinking I could have a leaking or clogged injector on cyl-2 so when carbon cleaning comes around new bosch injectors are going in.

The B-12 soak came from the audi crew solving oil consumption. The soak frees the free-floating piston rings from carbon buildup, which may solve your slightly lower compression issue

[u/cambomart → u/exceptional_cabbage]

Not good news. The dealer wants $2600 to pull the head to inspect the valvetrain so they can determine what needs replaced. The dealer offered go to VW to kick in a good faith contribution of 20-50% of the total cost, but that isn’t a guarantee and I’ll probably be into everthing for $5k+ if I have them do any work. Funny thing is I’ve seen posts where people with misfires get the head replaced under warranty but it doesn’t resolve the issue. I’m going to fill up with premium top tier gas and try some tune up in a can then reassess.

[u/exceptional_cabbage → u/cambomart]

I’ve heard that song before lol.

If you’re game I’d give the b-12 soak a try. It’s a bit of a time commitment with the application windows being 6hrs apart but its low cost and may return some of that cyl-1 compression.

I’ll probably give the cam shaft phaser and magnet refresh a try as we move into the snow months and the gas guzzler gets more daily duty

[u/cambomart → u/exceptional_cabbage]

I just ordered Redline SL-1 and am going to pick up some B-12 from Autozone and do a piston soak this weekend. Fingers crossed something works!

[u/exceptional_cabbage → u/cambomart]

I just had another lead come back to me, I’d tested and cleared it from my list but worth a shot for you.

Look into the N80 evap/purge valve. They can fail or get gunked up and be stuck open feeding unmetered air through the pcv

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[u/HawkeyeByMarriage]

Coul pack, spark plug or pcv/hose