I have a 2012 Yukon Denali with 180k miles that I have done a hardware AFM delete with an L9H camshaft. Every single startup it will show misfires on the misfire data in the scan tool but it doesn't feel like it is misfiring. I only knew it was doing it because on a true cold start if I put it in gear before the engine speed settles down (after running 1-2 minutes) the check engine light will start flashing. My wife and I both remote start the truck 95% of the time so we had no idea it was doing it until I was in a rush one day and fired it up and left almost immediately. It will show misfires on every single startup even if I turn it off and immediately turn it back on, it just won't set a P0300 unless I put it in gear on a cold start. The truck runs fantastic and hasn't done anything else weird. I sent the truck back to the tuner and he said everything in the tune looked good. I can't come up with any ideas that make sense that would affect it only on startup. I changed the front oxygen sensors just because I had been thinking about it anyway since the truck had high mileage and I also swapped a MAF from another truck and it made no difference. The misfires are generally focused on cylinders: 6, 7, and 8; but sometimes they're focused on cylinders 1 and 8.

The backstory: I was nervous to buy this truck because of the AFM and high mileage but it was so clean and it ran like a top. We had it a few months and my wife was driving and said the check engine light started flashing. It showed 2000 history misfires on cylinder 6 when I checked it. So I figured the AFM was going since it was cylinder 6. I had it tuned to disable the AFM to try and kick the can down the road. After the tune it started misfiring above 2k RPM (mostly on 6, 7, and 8...). So I pulled the trigger on doing the AFM hardware delete figuring the tune worsened the problem rather than making it better and the lifters were just done. I replaced the camshaft, lifters, timing chain tensioner/guide, timing chain, oil pump, spark plugs with TR-6, and then had the truck retuned.

 

It might need a valve job.

 

I thought of that but usually a valve issue will cause a misfire at idle as well which I don't have. I did a compression and leak down test on some of the cylinders (I think it was the main ones that show misfires but this was in November so I don't remember) and all checked out.

 

I don't know the specifics on the L9H camshaft but I have seen other 6.2Ls, specifically in 07 Escalades that while advertised as having VVT but no DOD, still using DOD lifters on cyls 1, 4, 6, and 7. The DOD enable switch just being set to Disable in the stock calibration. The engine shops I tune for when doing a DOD delete use older non DOD 5.3L cams and eliminate the VVT too, or use the 2010-13 4.8L cam which retains the VVT but never used DOD lifters in the 4.8L. It almost sounds like your problem might be what happens when you try to try to run non DOD lifters on DOD cam lobes. Just throwing this out there. Maybe others can correct me if I'm wrong.

 

If you use non-AFM lifers with an AFM camshaft the AFM cylinders will have low compression and it certainly won't run right. The L9H camshaft is the GMPP cam for AFM delete so nothing is mixed and matched.

 

I'm well aware of that. I'm just saying I do the computer work for a lot of local engine shops when they do DOD deletes and none of them ever use that cam. That's all.

 

What cam do they use? This was the only cam I found mentioned for the 6.2L other than aftermarket when I was researching the parts to buy.

 

I said what cams they use in post #4. I've also seen on 2 different 5.3s that the non DOD valley cover caused misfiring with noises like someone threw a bunch of rocks in the intake. Only putting the DOD valley cover back on would make it go away. That particular engine shop regularly puts non DOD valley comers on DOD motors with DOD lifters and cams still in them to solve low oil pressure issues. 99.9% of the time it doesn't cause any issues but for whatever reason on these 2 5.3s it just wouldn't work. I just mentioned it might be your cam choice because you said your misfires seem to be on some of the DOD cylinders. I know cyl 8 isn't a DOD cyl but sometimes the cyl either before or after in the firing order of the actual misfiring cyls will record misfires. And 8 is in between 1 and 7

 

And I've seen with my own eyes two 07 Escalade 6.2s with DOD lifters in them and the DOD switch set to disable in the factory calibration

 

I picked a random 2010 GMC Sierra with a 6.2L from my tune files and gave that VIN to my GM parts guy and he verified that 6.2L definitely used 16 non DOD lifters. Since what I gather the L9H started in 2009, I don't think your cam choice is your problem.