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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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Okay guys I bought this truck from a dealership in September. It was used and I got a 30 Day warranty. After the 30 day period, this is what’s happening. I got in my truck and noticed a rough idle. My truck would shake and out of no where the check engine light starts flashing and my traction control light comes on. I took it to my buddies and we got a P0308 Misfire Detected Cylinder 8. So I did some research.. I also had a code which was P219B. Air/fuel mixture ratio. 

 

I changed the spark plugs.. didn’t work. I moved coil packs.. the misfire didn’t move. So we watched Live data on the scanner and injector 7 was acting up. So we replaced all the injectors on that side and this truck is still missing.. I’ve ran seafoam through it and cleaned the throttle body and MAS.  My buddy things I have a valve issue or something wrong with the cylinder…
 

there is no AFM deleted or anything of that sort. The truck has worked flawlessly until now. 135,000 miles on it. The rocker arms all

looked great.
 

please someone give me something to try. I just want my truck fixed lol.  
 

posting a link below of what it’s doing…


 

 

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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

Posted Images

CamGTP Senior Enthusiast

CamGTP

Posted
Posted

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.

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CamGTP Senior Enthusiast

CamGTP

Posted
Posted

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 replaced all the injectors on the one bank but did you actually replace the cylinder 8 injector? Because cylinder 7 and 8 are on opposite banks.

 

 

  • Like 1
The Pig Dusters Enthusiast

The Pig Dusters

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We replaced all injectors on that side. It seemed like injector 8 was working fine but #7 was acting weird on the live data scanner.. so we replaced all 4 just in case. The weird thing is the truck idles like crap but drives amazing. 
 

I don’t have a scope is there any other way to see if the camshaft is working correctly?

newdude Senior Enthusiast

newdude

Posted
Posted (edited)
54 minutes ago, The Pig Dusters said:

We replaced all injectors on that side. It seemed like injector 8 was working fine but #7 was acting weird on the live data scanner.. so we replaced all 4 just in case. The weird thing is the truck idles like crap but drives amazing. 
 

I don’t have a scope is there any other way to see if the camshaft is working correctly?

 

So just to be clear, you replaced all the injectors on the driver's side so 1, 3, 5 and 7?  

 

If its the cam lobe, time to pull the head to pull the lifter.  

Edited by newdude
evad Enthusiast

evad

Posted
Posted

i would check for a bad pin connector in the main harness or the "green crusties" in the injector harness  

The Pig Dusters Enthusiast

The Pig Dusters

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  • Author
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Okay I’m sorry I have you guys confused. I have a misfire on cylinder 8. So if you’re looking at the front of the truck, it would be the furthest cylinder on the passenger side. We replaced all the injectors on that side of the engine. I’m not sure of the injector # correspondence but we replaced all the injectors on that side and still have a misfire. All the connectors looked good as well.  
 

it only does it at idle. I can drive it and it be completely fine.

 

 

86D8564B-8CBC-48F0-800D-EEC162FC38D5.jpeg

evad Enthusiast

evad

Posted
Posted

i had 2 trucks give me the same symptoms that you are saying and both had electrical probs. over by the main fuse box on top the fender. the rats nest wiring and plug may have a lose pin that causes funny ****** to happen. but idk i hope you get it figured out. i do think if you had a plugged cat you would know for shure

evad Enthusiast

evad

Posted
Posted

mine was the big plug in frt of the fuse box ....maybe the best place to start ...check the whole main engine harness for damaged wire also ...your gonna need a wiring diagram. Somebody on here might be able to help with a pin number for #7 and #8 injectors i don't have that info

The Pig Dusters Enthusiast

The Pig Dusters

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Posted

Interesting. But the issue isn’t the injectors anymore. They are all running fine on the scanner. But the truck is still misfiring on cylinder 8. I’ll search around and see what I can find. Thanks for sharing your story with your tuck though!!

evad Enthusiast

evad

Posted
Posted

oh sorry forgot to answer your second question.....the computer controls the injector pulse so a lose or wonky connection screws that up big time ...also you said in your vid that it goes away witch is suspect too but this is all my best guess so ..........?

The Pig Dusters Enthusiast

The Pig Dusters

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Posted

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.

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