Put a flywheel holding tool on it and use a wrench to turn the balancer bold back and forth. I wonder if maybe your crank is starting to let go. I've seen them break right in front of the 7 and 8 rod journal.
Replaced the Engine harness and still have the issue. My guess its a burned valve at this point. Everything else has checked out. I had the heads decked very slightly and they said a couple of the valves needed some work, that was done but thats all the info i have on it.
Ill go buy a 2016 Denali before I put a ton more money into this truck. Not sure what the shop will do to pinpoint the issue to even try and have an idea what it will cost.
can you pressurize the suspected cylinder to test for intake or exhaust valve leakage? is there a lobe wiped off the cam, are the rockers moving proper distance?
I'm thinking its a valve problem or lash. A plus number means that cyl is down on power and the computer is trying to make it up with fuel. If your sure the lash is good I'm think a valve not sealed or bent.
This has been a long time coming but the truck is now in the shop, I felt like I have done more then enough to figure this out and just don't have the time or space to keep digging in. So far they haven't found anything im pushing them to do a leak down and compression test on the #8 hole, then look into pulling the head off.