Sudden misfire and CEL

[bsimms89]

Just got some nice weather for the first time in months and went to take out my 2012 for the first time in probably 4 or 5 months. It started up great, I let it idle in the driveway for a few minutes to warm up and then started driving. It was driving great and sounded good and after about 5-10 minutes I got to an open road and started to give it a bit more gas, it started reving up and sounded good till about 3,500 rpms and then sounded like I was on the 2 step in my racecar, the CEL light started blinking and stabilitrack warning and everything else lit up. I immediately backed off the gas and turned onto a side road and it was idling rough.

I shut it off for a few minutes and started it back up, it sounded better but still a little off so I gently reved it in park up to about 2,000 and it seemed to smooth itself out. Then I started limping it back home, it started off the drive back ok and the CEL went away, then after a couple minutes the misfire came back and so did the CEL but steady this time , not blinking, and occasionally a pop in the exhaust which I'm assuming was unburnt gas hitting the hot header. Took me about 5 minutes to get home going gently.

I hooked it up to my code reader and had codes P0308 for a misfire in cylinder 8 and U0073, which I think happened when the whole dash lit up when the misfire first started.

The car is a 2012 SS with only 22,000 miles on it ( its my secondary car, a convertible, and I only drive it on good weather when its warm out). About 6 years ago I installed ARH long tube headers and exhaust, and a CAI intake and had a dyno tune.

My next step is I'm going to compression test the cylinder and if that comes back good then I'll replace all the spark plugs (they are about 10 years old) and wires and try starting it up and see what it does. If the compression is bad I'm guessing I'll have to take off the valve cover and check the valve spring.

Does anyone have any other advice on how to proceed?

Has anyone seen a spark plug suddenly go bad while accelerating like that. I didn't head any other noises when it started, I don't have and valvetrain noise. When I was accelerating it literally sounded like when I'm on the 2 step in my drag and it cuts the spark to limit rpms, so I'm hoping that's what happened is that it just lost spark in that cylinder. Would suck if I broke a valve spring (but I guess if the piston and cylinder was good but I needed a new head I would probably use it as an excuse to get ported heads and swap the cam, just not looking to spend the money on this car right now).

[Risingson05]

Ok just went through this myself . Injector or coil pack or plug or wire lol.
Try moving #8 cylinder coil pack and see if the misfire shows up on another cylinder .. did you use fuel treatment before storage in the winter ? ..

It also could be a bad ground cable or main harness . I ended up having a coil and them a ground cable causing mine , all in all yeah I would go ahead and change the plugs since their that old .



Annnnnddd wow !! You have a copo !!! That’s gotta be insane !

[Risingson05]

U0073 is a failed communication code . Po code stating misfire in #8 cylinder

Communication code makes me suspect a ground harness or ground or battery .

[bsimms89]

I think the U0073 code was a one time occurrence that happened when the CEL came on as all the lights on the dash came on at once, but after a few seconds they went away and only the CEL came on. I think it just happened when the computer got mad during the initial misfire and isn't a continuing problem, at least that's what I'm hoping.
I've got new plugs and wire and one new coil arriving tomorrow (even if I don't use it I'll have it as a spare). The only thing that worries me is how suddenly it came on, don't want to have to pull the head on this car right now.

Yeah I got the COPO in 2017 with the whipple, ran it like that for a while, in 2019 we switched the necessary body panels to turn it into a 2019 so we could run it with the Magnuson supercharger in stock, and last year we switched out for the 2020 heads as we had already had issues with the old heads sealing (had a burn through between 2 cylinders loosing compression) and a few people we knew wrecked due to coolant leaking through the head seal. So it was built as a 2017 but its setup as a 2020 now.

[Avenging Orange]

I would get an occasional pop/backfire on hard ecceleration.....but never a code. It turned out to be a plug that had what looked like a small chip of carbon literally melted to the tip of the electrode...thus causing a very narrow gap. Replaced plugs and runs like a champ. May not be your issue...but just a thought.
Good luck.

[SS Blinder]

I would check for signs of critters in the engine bay first. Then check the wiring and possibly the battery (old batteries have been know to go crazy). Good luck and I hope its easy

[bsimms89]

Just seemed to be bad gas that fouled a spark plug. I put 10 fresh gallons in it and changed the #8 spark plug and now all is well