2015 Sierra 6.2L Engine Issues - Random Misfire at 15k miles

mt_at · December 17, 2015

Hello everyone,

First time poster here, used to be a member of another forum that really wasn't too useful... I've lurked on here quite a bit and it seems like y'all do things a lot better!

Anyways, I replaced my 08 sierra back in May with a 2015 Sierra 1500 Crew/Short All-Terrain with the 6.2, and I've put about 15.5k miles on it since then, and have done some lighting/suspension mods too (pic attached)

I've loved the truck so far until yesterday. The day before I drove about 500 miles. It sat overnight (in about 20 degree weather) then I got in the truck the next morning to head to a meeting. About 1/4 mile down the road I started getting a flashing CEL, traction control off light (oddly no DIC messages like service stabilitrak or anything).

Truck was running VERY rough and ended up stalling out. I stopped and let it sit for awhile, checked the plug wires (heard of that issue on a friends truck) but everything seemed ok. Tried to start again... it cranked for a full 6-7 seconds (which is way long) and started barely, but sounded like complete sh*t, very rough and on the verge of stalling out (no CEL on, which was odd). Turned it off and called GM Roadside, got towed to the dealer.

Talked to the SA this morning, he confirmed "it sounded like complete sh*t" and is throwing the dreaded random misfire code. They called GM Tech Assist and apparently there has been a known fuse block issue of some sort, so that's what they're checking next - which seems like a lazy way to diagnose a misfire issue but whatever.

Wondering if anyone has experienced/heard of random misfires on a low mileage gen V 6.2? Really hoping it's nothing major, but I'm second guessing that $50k purchase of my first new rig already...

mt_at · December 18, 2015

Update from the dealer:

No clue what is wrong and are now working with GM Engineers.

Electrical/Ignition system Tests fine, but when running the truck it misfires all over the place, every single cylinder had approximately the same number recorded in the ECM... We know it's not bad gas, but they think it's fuel system related.

Still, any suggestions or similar experiences (if any) are welcome!

02Sierra_Mark · December 18, 2015

Hey mt_at - I don't really have any suggestions or similar experiences but I do hope they get your truck fixed quickly. Good luck man!

TJay74 · December 18, 2015

I would let them diagnosis it, no one on here is going to be able to armchair QB this one. The GM tech engineers are pretty smart, they know the systems inside and out so as long as the dealership is willing to put the time into the issue GM will get it resolved.

BuyAmericanIron · December 18, 2015

TJay is right, let GM and your dealer take the time to fix it right. Probably nothing major.

My 15 6.2L had a similar issue. It started running very rough shortly after leaving home one morning. Limped it missing badly for nine miles to work. Diagnosed as a failed #3 fuel injector. Injector did fix the misfire. When I got it back, it was disappointing to find a nice chip in the paint on the LF fender. I could easily see where they tried to rub it out but mysteriously forgot to mention it to me when I picked it up. Good luck with your repairs and look your truck over closely before you move it at the dealer. Just my $.02 cents.

mt_at · December 18, 2015

Thanks for the input...

The truck is at a dealer a friend owns and that I used to work for, so I know all of them well and am confident in how they treat vehicles luckily.

If it was a bad injector then it should only be the one cylinder misfiring, at this point everyone wishes it was that simple!

Everything they've tested so far has come back normal, and all 4 of their master techs looked at it today. Fuel pump is the first on the list tomorrow (which I think there was a TSB for - if someone has access to it that would be awesome) but so far nobody has a clue.

UPDATE: time to tear down the top of the engine. Stopped throwing misfire codes, now throwing MAF and MAP. Tried replacing both parts to no avail, now it's only throwing map but misfiring still. Valve springs are next up, ignition and fuel systems all working right.

pm26 · December 20, 2015

Unfortunately, random quality control often results in random problems that are so difficult to find. Do they suspect that the AFM crap is causing this? I bet the problem is of electrical origin, mostly likely a bad connection somewhere in the fuse box.

mt_at · December 21, 2015

Electrical and Fuel systems are working right. Wasn't making noises that would indicate a lifter issue which is the AFM problem, the only thing now would be valve springs or something.

mt_at · December 24, 2015

Got a call from the dealer - they found the problem. Two bent valves, which is really bizarre.

GM is flying 2 engineers out from Detroit to look at the engine after Christmas since it was such a weird issue, but they want to make sure the rest of the engine is ok and it's not gonna be an issue with other engines...

pm26 · December 26, 2015

Got a call from the dealer - they found the problem. Two bent valves, which is really bizarre.

GM is flying 2 engineers out from Detroit to look at the engine after Christmas since it was such a weird issue, but they want to make sure the rest of the engine is ok and it's not gonna be an issue with other engines...

Well, there are known valve issues with Corvette engines, could this be a similar problem?

wrc7732 · December 26, 2015

Would have thought they would've done a compression check before tearing it down and would have identified a bad cylinder or more that way. Would've helped in know which side to tear down.

Very odd though to have two bent valves in such a low mileage truck. What's the corvette valve issue the previous poster mentioned?

Sent from my iPhone using Tapatalk

pm26 · December 27, 2015

http://www.z06vette.com/forums/f62/gm-response-failing-ls7-engines-125371/

mt_at · December 29, 2015

Got back to town and picked it from the dealer yesterday. Apparently it was bad valves and spring on cyl 8 exhaust.

Drove about 50 Mi and the CEL is back on. My crappy old code reader doesn't work too well on these trucks, but it looks to be a Catalyst system failure. So going back in today.

mt_at · December 29, 2015

UPDATE: 3 catalytic converter codes for bank 2. Dealer has ordered, and is going to replace the Cat. Guessing they ran the engine enough while it was misfiring for it to go bad.