2021 Custom Trail Boss 5.3L L82 6speed Misfires and Dash lights

CSSM2021 · October 30, 2021

Vehicle Build date 09/21

Engine Julian date 08/16/21 Monday 1st shift

Only 500kms on truck. Vehicle was off and parked and when started it was running really rough aggresively shaking, and dash showed Service Parking Brake,Service ESC, didn't notice Check Engine light on or flashing at all.

Turned it off and after 3 minutes turned on as was running fine.

Took it to dealer to check it out as just purchased 2 weeks prior and was told showing P0300 code and data showed cylinder 4 had about 2000 misfires. They tried to use freeze frame data to see if could get issue to occur again but couldn't. They checked out spark plug 4 and all seemed fine. Couldn't find any issues. Decided to take it upon myself to monitor it as I have an ELM327 with TorquePro app and am noticing misfires occuring on all cylinders occasionally but don't really notice that much physically when driving. Cylinder 4 however seems to misfire the least now, so was thinking it was a loose plug but not sure. Seems like gas is being used a lot more but could be wrong on that. Not sure if normal to show a few misfires here and there but doubt it.

Will probably bring back to dealer as soon as I can prove something as don't think they will do much just yet.

Just curious if any others out there with same recent build on truck and engine having issues. Don't believe it is lifter or valve issues as my truck is way out of the dates in those bulletins. Thinking maybe injectors...thoughts?

redwngr · October 30, 2021

Does tow/haul mode change the misfire pattern the TorquePro app is indicating?

CSSM2021 · October 31, 2021

So I took it to the store this morning and ran it in tow/haul mode and monitored it and same results...minor misfires in different cylinders as follows:

Cyl 1 had 2 misfires

2 had 1

3 had 0

4 had 1

5 had 6

6 had 2

7 had 2

8 had 1

After leaving the store I drove across the street to another store and shut it off and went in. Got what I needed, 5 mins later came out and started it. Noticed rough idle and watched TorqueApp and it showed cylinder 4 having 260 misfires (same cylinder that had 2000 when brought to dealer with the more aggressive shaking idle and the dash lights on) and minor counts again on some other cylinders. No lights this time though and just minor chugging. Revved engine higher and seemed to stop.

Drove home while still scanning and just the minor misfires.

CSSM2021 · October 31, 2021

BTW, when the 260 showed upon startup I was not using tow/haul mode.

CSSM2021 · November 22, 2021

Update...issue occurred again and same lights tripped along with CEL quick flash.

Brought to dealer, found fuel injector 4 bad and replaced. Engine sounds better now. Still monitoring it though and al cylinders minor misfires every drive...not sure why.

Dealer can't do much without code tripping....

Kilgore Trout · November 22, 2021

I haven't looked on my new AT4, but on my old 2007 Yukon (bought new, well into six figures mileage wise now, 5.3L LMG), occasional random misfires are quite common (single digit low). Anything into high double digits or more misfires, meant something was amiss (no pun intended),

Edited November 22, 2021 by Kilgore Trout

1997SierraSLT · November 22, 2021

GM can do some things well. Reliable electronic fuel injection systems is not one of them. You're going to see that flashing CEL more times than you care to imagine over the life of ownership.