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

The Pig Dusters
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The Pig Dusters Enthusiast

The Pig Dusters

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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?

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CamGTP
CamGTP

Compression and leak down test is the next step if it was me.   I assume you checked for a broken valve spring if you had the valve covers off.

CamGTP
CamGTP

Worst case is that it's a smoked camshaft lobe.   Probably need an ociliscope to make sure everything is working like a normal like a non issue cylinder.   I noticed you say you re

newdude
newdude

This symptom set sounds exactly like the state trooper 2018 Tahoe 5.3 we had in last year.  Cam was toast on cyl. 8.  Had good plug, wire, compression and leakdown test.  Idled like crap and when you

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newdude Senior Enthusiast

newdude

Posted
Posted (edited)
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 by newdude
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The Pig Dusters Enthusiast

The Pig Dusters

Posted
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Posted
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 Senior Enthusiast

pokismoki

Posted
Posted (edited)

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 by pokismoki
  • 2 months later...
Alberto New Member

Alberto

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Posted
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

  • 2 weeks later...
The Pig Dusters Enthusiast

The Pig Dusters

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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!

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  • 11 months later...
Bentley Enthusiast

Bentley

Posted
Posted (edited)

I am having similar problem but with it the following occurs:

I get the cylinder 8 misfire on idle, then hear a click and the oil pressure spikes.

Then engine light etc

 

 

Thoughts?

 

 

Edited by Bentley
The Pig Dusters Enthusiast

The Pig Dusters

Posted
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15 hours ago, Bentley said:

I am having similar problem but with it the following occurs:

I get the cylinder 8 misfire on idle, then hear a click and the oil pressure spikes.

Then engine light etc

 

 

Thoughts?

 

 

Yes we found the issue. We repaired the valves and the truck ran good just for about a month then the same stuff started happeneing again. I finally decided to let my buddy do a full teardown. We inspected the cam lobe using a camera which is fine in the moment but we decided to pull the cam and we found the lobe on cylinder 8 was worn just enough to cause a misfire. The lifter had damaged to the roller but it was still functioning. Prob because of bad oil changes from previous owner. We put a new cam and all 8 lifters in and she works great now.

The Pig Dusters Enthusiast

The Pig Dusters

Posted
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On 1/4/2023 at 9:00 PM, Alberto said:

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

Cam lobe was worn just enough to cause a misfire. We didn't see it the first time because we used a camera. 

The Pig Dusters Enthusiast

The Pig Dusters

Posted
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14 hours ago, Alberto said:

1 of mine had the same issue and the other truck had a broken spring.

I wish mine was a broken spring lol. 5k for new cam and lifters. I love my truck though so I guess it was worth it. 

The Pig Dusters Enthusiast

The Pig Dusters

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34 minutes ago, Alberto said:

The one that was the cam came out to be 8500 for the DOD delete, but same here I like the truck.

 

How is the delete? I didn't do it and I don't use my Range AFM device either. It made it shift hard and just act weird. I keep up on my oil changes and seafoam every so often to keep her clean on the inside.

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