Cylinder Misfire

cheerdude63 · December 23, 2022

Hello all, looking for some help here. I am lost and feel as though I have exhausted easy options. I do not have a scan tool powerful enough to run tests, like balance test, injector flow rates and stuff like that.

I have a 2015 GMC Sierra, L83, about 107k miles on it. Last winter, so 2021, I developed on cold mornings a misfire cylinder 1 and 2 mostly, it would be very random. Some times I would drive the truck everyday for a week with no issues at all. Then one morning I would fire it up and it would miss very badly, enough to flash the CEL and turn of traction control. As the weather got warmer the problem subsided and I hadn't had the problem at all. So not at all that I completely forgot about the problem until it started getting cold again. It started doing it randomly again, like once every 5 days or so. I had a broken exhaust manifold and wile I was changing it out it found some bad, like really badly cracked wires. I thought HORRAY, I found the problem! I changed the wires and the truck ran great for a week or so. Took it on a trip about 1000 miles with no problems at all.

Then it started happening again. I had to put coils on the truck at about 55k miles so I thought what the heck, maybe the coils are bad and chalked it up to that. I bought 2 coils, replaced #1 and #2 coils, decided I would check the plugs while I was at it. Pulled the plugs and they were fine, had about 50k on them because I changed them when I did the coils. I put 2 new ones in anyway just to eliminate the problem. Fired the truck up and it was all good for a few days. I drive the truck almost everyday, I fired it up for work one morning, about 35 degrees out or so and there it was again, bad # 2 misfire. I had some time before going to work so I swapped coils from #4 to # 2. Fired it up, problem went away, no misfire at all, all the way to work. Next morning, same thing fired it up P0302 again!

So now I have eliminated the plug being the problem, the wires are new, and the coil is new and swapped with a different cylinder and the misfire stayed at #2. It's still random though. Just this week, Monday and Tuesday, misfired the whole way to work, full throttle pulls, NO misfire count, light medium loads and it chugs like an SOB. Wednesday morning fire it up, runs perfect, not a single misfire the whole way to work, to lunch and back, or back home. This morning, really cold -4, ran like crap all day. I started it after work, missing at idle and moved it to the front of the building to let it warm up. I had to grab my keys, I shut it off, let it sit maybe 5 minutes. Fired it back up and it ran perfect the whole way home.

I am seriously lost with what this could be however, I do have some thoughts that maybe some of you might have a response to. I notice on the days that it runs well, it starts and immediately idles at 750 rpm or so after the initial start, no matter the temperatures. The days that it runs like crap and misses, it stats and goes to high idle like 1200 rpm or so and doesn't drop back down for 5 or 10 minutes or so. Sometimes, if I shut the truck off and re-start the miss fire goes away. I am almost thinking I have a programming issue. Is it possible to lose it's parameters like if I had a bad battery or replaced a ground or something, maybe a bad positive battery cable? I did replace the negative side when I did my manifold because it was corroded badly by the manifold. I can't think that this issue is mechanical like a bad lifter or anything because it runs hard at WOP throttle with all the beans and the misfire come out of nowhere on some days. The truck has been tuned and DOD is disabled in the ECM and has been since the truck had about 1200 miles on it. It's always ran great.

Any helpful ideas would be appreciated at this point. I am not sure what to do.