2015 GMC Sierra 6.2 P0308 Misfire Cylinder 8 CANT FIGURE IT OUT

[The Pig Dusters]

29 minutes ago, newdude said:

 

 

 

 

I'd run and get a lotto ticket LOL.  Talk about luck.  

 

What is crazy is you were showing the miss on 8?  And 8 (passenger rear most cylinder) is a non active cylinder so installing the Range wouldn't fix a miss on 8.  That would mean 1, 4, 6 or 7 were the problem, 7 (driver's rear most cylinder) being the most likely one it seems.  

 

I'd still keep in mind that there's still an "if" scenario where if an AFM lifter has already stuck once and seems to have re-set, that it re-sticking can't 100% be ruled out.  

Dude I Know lol. So Is it possible one of the other cylinders was causing the 8th cylinder to misfire? What is weird is when I took it to the dealership, the technical guy told me cylinder 8 was a AFM cylinder. Which I know isn’t right. I agree that 1, 4, 6, and 7 are AFM cylinders. 
 

I want to ask this as well. When the truck goes in V4 mode, the lifter collapses right? Is it possible a lifter can get stuck in the uncollapsed position?

[newdude]

54 minutes ago, The Pig Dusters said:

Dude I Know lol. So Is it possible one of the other cylinders was causing the 8th cylinder to misfire? What is weird is when I took it to the dealership, the technical guy told me cylinder 8 was a AFM cylinder. Which I know isn’t right. I agree that 1, 4, 6, and 7 are AFM cylinders. 
 

I want to ask this as well. When the truck goes in V4 mode, the lifter collapses right? Is it possible a lifter can get stuck in the uncollapsed position?

 

 

They will typically get stuck in the collapsed position.  If they are stuck open, that would lead me to believe that the oil control solenoid for that cylinder in the VLOM is not sending oil to the pin to unlock and collapse the lifter.  

 

I can see a tech thinking 8 is only because the newer 5.3s and 6.2s with DFM (dynamic fuel management) have active lifters in all 8 cylinders instead of just 4 of them.  Possible confusion on their part.  

 

Clean oil and good filtration are key with this system.  

 

 

Edited October 17, 2022 by newdude

[The Pig Dusters]

33 minutes ago, newdude said:

 

 

They will typically get stuck in the collapsed position.  If they are stuck open, that would lead me to believe that the oil control solenoid for that cylinder in the VLOM is not sending oil to the pin to unlock and collapse the lifter.  

 

I can see a tech thinking 8 is only because the newer 5.3s and 6.2s with DFM (dynamic fuel management) have active lifters in all 8 cylinders instead of just 4 of them.  Possible confusion on their part.  

 

Clean oil and good filtration are key with this system.  

 

 

Ohh I see now. Well that would make sense. I’m gonna keep driving it and hope to have no more issues with it. I’m getting the oil changed next week and I’ll have them check for any metal shavings just to be sure. My buddy also told me that since I was having that misfire, the cylinders could get very dirty with carbon build up. What cleaner would you recommend running through my engine to help with any of that stuff?

 

I used Seafoam once like a month ago but just curious on your thoughts. 

[pokismoki]

just a cheap shot in the dark... reflash the engine ECM with stock tune file.  assuming you did a tune . but i would erase it and start fresh, wether its tuned or not.

 

 

i remmember when my truck was newish 10K miles i would get some lights flash on the dash, stability control error message. i ignored it, seems to have went away and no CEL codes on her

Edited October 19, 2022 by pokismoki

[Alberto]

On 10/8/2022 at 12:23 PM, The Pig Dusters said:

Ohh okay gotcha. Yeah as soon as I start driving, the flashing light will stop, and it will

drive smooth as silk. But when I’m parked or idling at a light or drive thru, it misses so bad and shakes.

Did you find the issue? I have 2 trucks doing the same exact thing

[The Pig Dusters]

Update. I wanted to share what all has happened over the past few months. The truck ran great for about 2 weeks then the misifre started to really show itself at idle. So my buddy and I said screw it and took it apart to see if it was the dreaded lifter or camshaft causing this. We found a functioning lifter on cylinder 8... and we found no damage to the lobe... What we did fine was a burnt exhaust valve. The head wasn't warped so we did a deep clean and replaced the valves and wouldn't you know... No more misfire and a perfectly running truck!