2015 GMC Sierra 6.2 Weird Misfire Issue

diamondwhite_6.2 · June 29, 2024

Hello All,

I have a 2015 GMC Sierra. 6.2l 8l90. 4wd. Having a very very weird misfire that we CANNOT diagnose. Some more background on the truck. We just finished installing a Jasper Reman 6.2, as the original engine developed a rod knock, 2 days after getting it back from having the collapsed lifter replaced and deleted. The truck was repaired with a BTR DOD deleted, BTR valve springs and Stage 1 truck cam. Tuned very well for 93 by a prestigious shop. Aside from what I have listed the truck is stock other than a muffler and resonator delete.

The day after I got the truck back(after putting roughly 200 miles on it driving from the shop and back home) I went for a speed run to experience the deleted 97mph limiter. The truck pulled hard until about 105mph and felt like it had no more power, so I let out. Went and finished my errands and when I got home I noticed it was knocking. I took it to my buddies shop to inspect and after dropping oil pan and pulling caps, cylinder 4 spun and overlapped and pushed it's way into cylinder 3. Rather than replacing crank, 2 rods, block, etc, we ordered a Reman l86 from Jasper. It already comes DOD deleted from them, so we installed my stage 1 truck cam, lifters, valve springs and cam gear with 4* Phaser limiter.

Skip to getting the engine put in, new radiator(to not take the risk of the oil cooler being contaminated with bearing material) and hooking everything up and filling with fluids. We were ready for first fire up. It took a few tries to get the fuel back into the rail. It ran and idled smooth. After about 3 months it was nice to hear that slight chop from the cam. Backed it out of the shop, put it in drive to head home, and it started misfiring very bad..

Did a drop pressure test on the injectors and cylinder 8 read 5.5psi. So we replaced that injector and now they're all between 27-42psi. And it helped. It now with neutral rev clean. But if I drive it at all, it's misfiring randomly. During a WOT pull, it will put you back in your seat, misfire it's way through 2500-3000, pull between 3000-3500, misfire between 3500-4000 and so on. When it misfires, it sounds like a DSG transmission shifting(if you can relate the sound.). We triple checked timing and it's good. It seemed like the actuator on the cam gear was too easy to turn so we replaced that and installed my phaser limiter but now waiting on a new cam gear bolt. Not too hopeful that will fix it, so we're looking for ideas. All grounds are good. Fuel pressure pre/post of the rail is good. Data logged and when it's misfiring, it's knock retarding like crazy. 02 sensors are all over the place. The person I had tune it, has misfire monitors turned off so we are unable to see what cylinder it is happening on. Temp checked each cylinder and cylinder 8 is the coldest. Not cold as it's still firing but the coldest of all 8.

Has anybody experienced anything similar? What should we try next. It wasn't doing this before it started knocking. I've never felt it before which makes me believe it is nothing in the tune.

rlcole321 · July 1, 2024

A lot to deal with, I think you'll just going to have to have you buddy with the HPTuner ride along with you and look at live data to try to find it, could be fuel or timing related, could be the knock sensor system going haywire, or the traction control going bonkers. Usually when get into that much mod's on your vehicle, you should get your own HPTuner system and learn how to use it, I did.