91RS

iTrader: (8)

Sorry for the length but this is pretty thorough, I think. I have a 2012 Yukon Denali with 180k miles on it. Purchased it 6 months ago and it's been great until a few weeks ago. My wife called me and said the check engine light was flashing and while we were on the phone is stopped after about a minute (I didn't ask if she could feel it actually misfiring). I checked it when I got home and it had 2500 misfires on cylinder 6 and no other cylinders. I knew when I bought this truck I'd likely have to do a cam and lifters at some point but I was hoping to get at least a year or two out of it first. I decided I would have it tuned and disable the AFM in hopes of kicking this can down the road, I was thinking about getting it tuned anyway. I had a performance tune done to it rather than just disabling the AFM, I used a well regarded local tuner who I've used before for my cammed SS and my 08 Escalade.

The following day I drove it to work so I could see how it did and I got off the highway and when I accelerated from the off ramp I noticed the check engine light flashing but it didn't feel like it had misfired at all. I had inadvertently ran it lower on fuel than we ever have since we've had it so I thought maybe because I was turning and accelerating rather aggressively that it sucked up some air or trash floating around in the bottom of the tank. I filled up and on the drive home the check engine light started flashing when I got on the interstate and was on and off the whole way home. I drove it again the next day and same story the whole way too work. Misfire history showed 6700 misfires on cylinder 7, 1200 on cylinder 6, 1000 on cylinder 8, 660 on cylinder 5, 250 on cylinders 3 and 4, 120 on cylinder 1, and 30 on cylinder 2. I'm still not sure that I actually felt it misfiring. I swapped the coil, spark plug, and wire with no movement. Compression test showed ~200psi, dynamic compression test was 60psi at idle and ~100psi snap, fuel injector balance test passed, fuel pressure was good. I pulled the fuel rail to see if there was any trash in it anyway (nothing really noticeable) and swapped cylinder 7 injector with cylinder 1. I performed GM's injector cleaning procedure mixing top engine clean and gasoline and running the truck off it. No change.

At this point I was left with the tune being at fault or the lifter is collapsing under certain conditions. While driving home that day I noticed it would not misfire at all below 2000 RPM or WOT. I could power brake the hell out of it and get it to misfire sitting still and I attempted another dynamic compression test and got 200psi, no idea if that can even be done like that but whatever. I also noticed when power braking it that sometimes cylinder 7 would have the most misfires and other times it would be cylinder 8 followed by cylinder 6. Very odd. I changed the spark plugs to NGK TR6 since I was thinking about it anyway and it made no change. So I decided I'd reprogram the ECM to stock and see what happened before I started ordering parts and to my surprise it no longer showed misfires. I'm still not sure it was actually misfiring and I'm also pretty sure the engine sounded normal, even when power braking it.

I got with the tuner and he said he's done thousands of these and never had a tune cause misfires. He said his experience is that sometimes tuning high mileage AFM engines causing weird things to happen and recommended I just delete the AFM mechanically. He reloaded the tune today and enabled the AFM to see if that was somehow causing it but I still got the check engine light to start flashing on the way home keeping it above 2000 RPM. He said he made no changes to the factory misfire detection. I don't doubt him because he has a great reputation and my other two vehicles run fantastic but I really want to know why it's doing what it's doing wherever the problem actually lies. I'm not against deleting the AFM as I knew the day would come eventually (I didn't want it to be so soon though) I just want to make sure that's actually the issue here before I spend a bunch of money I'd rather not spend right now. I know this truck has some oil consumption as well so I'm considering swapping the engine for a used L9H to save time and fix both issues so that's an even larger amount to spend.

Thanks for any help.

91RS

iTrader: (8)

I guess I'll just delete the AFM entirely then and hope he's right. This is the list of parts I came up with if it helps anyone:

L9H Camshaft: 12623065
Cam Thrust Plate: 12589016
LS7 Lifters: 12499225
Lifter Guides: 12595365
L92/L9H Valley Cover: 12598832
Crank Bolt:
Head Gaskets: 12610046
Head Bolts: 19258707 & 12558840
Camshaft Bolt: 12682000
Timing Chain: 12646387
Timing Chain Tensioner: 12626407
Oil Pump: 12612289 or Melling 10295
Intake Gaskets: 19207929
Oil Pan Gasket: 12612350
Front Cover Gasket: 12633904
Water Pump Gaskets: 12630223

00pooterSS

iTrader: (40)

Had a similar issue with a 2013 Yukon denali. Came in running bad, weak, and no oil pressure (sending unit was bad)

I never was able to find or FEEL a misfire yet it kept giving me misfire codes, but only P0300. I spent a few days messing with it, compression was amazing on all cylinders 210-220 on all if I recall correctly (was over 200 on each and under 220), plugs wires etc looked great.

To try to get the misfire codes to go away I inspected and or replaced

Intake gaskets (needed anyway because oil sending unit was bad)
Plugs and wires ( was due by time anyway, had 100k on it)
Removed and cleaned all grounds and reinstalled. Ohmed them out while I was at it. No problems found
Performed crank/cam correlation a few times
Performed top end engine clean, and put in good injector cleaner in the tank
Had customer fill tank with 93, she had been running 87
Cleaned throttle body

After all of that and not charging her she didn't come back. So I never found out what the issue was. But it was a similar situation.. all except this one didn't actually misfire.


Keep us updated on what you find. I wished I could have had more time with that Denali but the customer refused to leave it more than about 2 hours at a time and for whatever reason after leaving it 3 days 2-3 hours at a time was upset that all of the free work we did hadn't fixed it so I assume she took it elsewhere.. She would always show up without warning asking "IS IT READY YET". That's fine, they can have her, but damn I wanted to know what was up with that truck.

91RS

iTrader: (8)

That's what's so odd about it, otherwise the truck runs fantastic.