P0300 multiple misfire

CFM · 04-12-2019, 07:00 PM

Just thought I'd post this for others experiencing misfires with a flashing check engine light and DTC P0300 set. There are many possible causes for misfires, but this was the issue in my case.

My car has been parked in my heated garage for the winter. I have started it once or twice to move it a few feet. I recently fired it up and it ran very rough with misfires and backfires. Logging showed similar misfire counts on cylinders 2 and 6. I checked all the usual suspects: wiring, coils, injectors, MAF, AFM, etc. No issues found.

I had to remove the intake to check the injectors and injector wiring. Intake runners looked oily. I left the intake flipped over on the bench while I was checking things out. When I got around to re-installing it I noticed oil was running out of the throttle body, so there must have been quite a bit in the intake. Of course the last thing a I checked was the plugs as the car has only 2600 miles on it. They were all heavily sooted. Cleaned them up and re-installed them.....no more misfires.

I had a similar issue last spring after the car had been sitting, but the misfire was isolated to only 1 cylinder. I took it to the dealer and the diagnosis was a backed out terminal on an ignition coil connector, which is why I started with the wiring this time. Now I am thinking it was more likely the same issue as this year.

Not sure if this is all PCV/oil ingestion related, or also due to the new DI cold start strategy whereby injection pulses are doubled for the first 60 seconds to speed up cat converter light off. This strategy also results in a very sooty exhaust for the first 60 seconds, as evidenced by the black mess on my garage floor, and may explain the complaints of a rough idle and black smoke on cold starts. See attached cold start TSB.

avalonandl · 04-14-2019, 08:34 AM

Run some techron thru it. The injectors are sticking.

Gunkk · 04-14-2019, 01:36 PM

Glad to hear you were able to diagnose and fix the fouled plug misfire.

Unfortunately the black mess on the floor is normal for our cars. It's from the weep holes in the muffler letting out condensate. It also makes for a pair of lovely stains on the muffler.

joeyazzata29 · 05-03-2021, 03:01 PM

My 21 SS 1LE Will only p0300 on the race track at track days.. limp mode, studdering shaking, flashing CEL, service rear axle etc.

You disconnect the battery and it’s fine, until you go back on track and it happens again

Couldn’t get it to do it on the street tho even with a bunch of hard pulls?

04-14-2019, 08:34 AM	#2
avalonandl Drives: 2019 ZL-1 Join Date: Feb 2009 Location: Troy, MI & Naples, FL Posts: 2,019	Run some techron thru it. The injectors are sticking. __________________ 2019 ZL-1 Acquired 4/23/21 at 6300 miles. Riverside Blue, A10, PDR. Traded in 2017 2SS with 6M and 32k miles. Continental Extreme Contact Sports. Now has 10,000 miles... 2012 Yukon XL Denali w/ 6.2 AWD

04-14-2019, 01:36 PM	#3
Gunkk Thank you Al Oppenheiser! Drives: Red Hot A10 ZL1 Convertible Join Date: Jun 2017 Location: Sarasota, FL Posts: 4,973 View My Garage	Glad to hear you were able to diagnose and fix the fouled plug misfire. Unfortunately the black mess on the floor is normal for our cars. It's from the weep holes in the muffler letting out condensate. It also makes for a pair of lovely stains on the muffler.

05-03-2021, 03:01 PM	#4
joeyazzata29 j1ss Drives: '21 2SS 1LE Join Date: Dec 2019 Location: Florida Posts: 384	My 21 SS 1LE Will only p0300 on the race track at track days.. limp mode, studdering shaking, flashing CEL, service rear axle etc. You disconnect the battery and it’s fine, until you go back on track and it happens again Couldn’t get it to do it on the street tho even with a bunch of hard pulls?

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