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

The Pig Dusters
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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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The Pig Dusters Enthusiast

The Pig Dusters

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Please let me know what you figure out. It’s so weird how it drives perfectly fine but idles like crap. I’m gonna check my  battery wires such as the ground and stuff today for damage.

 

so far I’ve replaced the injectors, swapped coils, clean throttle body, ran seafoam through it many times, clean MAS, replaced spark plugs and wires. We even smoked it and can’t find any leaks. It makes me more mad that it’s on one cylinder that performs perfect fine and has great compression.

newdude Senior Enthusiast

newdude

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Posted (edited)
On 10/8/2022 at 3:23 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.

 

 

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 picked up the RPM everything was great.  

 

I think its time to pull the head and lifters on #8 to check that camshaft...

Edited by newdude
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The Pig Dusters Enthusiast

The Pig Dusters

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Hmm. Wouldn’t I be having more issues than just one cylinder if the camshaft lobes were bad? I’m not a mechanic I’m just trying to understand lol. I also had a guy tell me to order the AFM delete issue as it could be something going on with that AFM lifters. He said he plugged that in and it fixed his rough idle and misfiring issue. I thought I’d try that first before anything else. I don’t wanna put crazy money in it if I don’t have to. 

newdude Senior Enthusiast

newdude

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5 hours ago, The Pig Dusters said:

Hmm. Wouldn’t I be having more issues than just one cylinder if the camshaft lobes were bad? I’m not a mechanic I’m just trying to understand lol. I also had a guy tell me to order the AFM delete issue as it could be something going on with that AFM lifters. He said he plugged that in and it fixed his rough idle and misfiring issue. I thought I’d try that first before anything else. I don’t wanna put crazy money in it if I don’t have to. 

 

 

1, 4, 6 and 7 are the AFM cylinders.  You are missing on 8 so a Range AFM won't help and still wouldn't help if the cam was trash on one of those 4 cylinders.  Could it allow an AFM lifter to somehow magically re-lock properly?  I suppose, but unlikely.  

 

With RPM the cam is spinning faster so the roller on the lifter is able to overcome the bad surface but when its idling its not letting it open the valves all the way.  

Edited by newdude
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The Pig Dusters Enthusiast

The Pig Dusters

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3 hours ago, newdude said:

 

 

1, 4, 6 and 7 are the AFM cylinders.  You are missing on 8 so a Range AFM won't help and still wouldn't help if the cam was trash on one of those 4 cylinders.  Could it allow an AFM lifter to somehow magically re-lock properly?  I suppose, but unlikely.  

 

With RPM the cam is spinning faster so the roller on the lifter is able to overcome the bad surface but when its idling it’s not letting it open the valves all the way.  

Ohh I see. Okay well tomorrow or Wednesday my buddy and I are gonna check one last thing. We did have the valve cover off to check the springs but we never did check to see if I could be dealing with a collapsed lifter on the cylinder 8. We never really ruled out a bent pushrod or something but if that is the case, I expect the camshaft to of taken some damage as well. I’ll update once we check. 

The Pig Dusters Enthusiast

The Pig Dusters

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Yes. So my truck was only missing at idle. We tried new plugs, wires, injectors, and we made sure of all the small things. No intake leaks or anything at all. I took it to a dealership and they told me it was a lifter. They wanted 5K FROM ME which I’d be okay with but the guy couldn’t explain how the lifter was only messing up at idle. So I said no and I bought a Range AFM disabler. It fixed my issue. I’m not sure how it fixed it but it did. No more misfire and no more flashing cel or stabiltrak.

 

 

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CamGTP Senior Enthusiast

CamGTP

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The AFM disabler turns off the function of the VLOM that control the AFM cylinders, but kinda doesn't make much sense because it doesn't work on cylinder 8.

Edited by CamGTP
The Pig Dusters Enthusiast

The Pig Dusters

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9 hours ago, CamGTP said:

The AFM disabler turns off the function of the VLOM that control the AFM cylinders, but kinda doesn't make much sense because it doesn't work on cylinder 8.

Exactly so I’m as confused as you are. 

newdude Senior Enthusiast

newdude

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Posted (edited)
18 hours ago, The Pig Dusters said:

Yes. So my truck was only missing at idle. We tried new plugs, wires, injectors, and we made sure of all the small things. No intake leaks or anything at all. I took it to a dealership and they told me it was a lifter. They wanted 5K FROM ME which I’d be okay with but the guy couldn’t explain how the lifter was only messing up at idle. So I said no and I bought a Range AFM disabler. It fixed my issue. I’m not sure how it fixed it but it did. No more misfire and no more flashing cel or stabiltrak.

 

 

 

4 hours ago, The Pig Dusters said:

Exactly so I’m as confused as you are. 

 

 

 

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.  

Edited by newdude

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