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Cylinder 7 misfire mystery on 08 silverado


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Hey guys. Im new to to this forum. I will try my best to describe my situation as accurately as possible. Thanks in advance for your help!

 

About 2 months ago i was driving home from work in my 2008 chevy silverado 1500 6L v8 when i started to hear a slight knocking sound coming from the engine about once every 20 times i tried to accelerate. I didnt think much of the sound until about 3 days later when i was driving and all of a sudden it started running really rough. Rough idle, poor acceleration amd lots of vibration commimg from the engine. So within the next day i purchased a obdii scanner and scanned the flashing engine light. The p0307 code popped up. Cylinder 7 misfire. From what i had heard this problem was usully easy to fix. So i went ahead and started changing things. I changed the coil pack, spark plug and injector for that cylinder. Nothing changed. So i did a compression test on the cylinder and it was reading between 0 and 30psi. That scared me a bit. So i took it to gm and after and inspection they told me i had a cylinder 7 misfire and to tell anything further they would have to take the engine appart. So thwre went $100 for nothing. So next i took off the cylinder head to see what was up. The cylinder itself looked ok, no damage. The piston was ok the one exhaust valve was black with carbon build up. So i decided to take the head to a machine shop to get it resurfaced and get a valve job done. He ended up replacing one of the valve guides but it wasnt the 7th cylinder one. So we put the head back on. Brand new gaskets and head bolts and everything. Got her back together started her up and same problem but it was as bad now. Weird that the problem got better but wasnt solved. We then checked the compression again and it was at a steady 120psi. Now i was confused as to what it could be. I checked the o2 sensors and they looked clean. There was no rotten egg smell comming from the exhaust so i assumed the cats were fine. Then i started playing around under the hood while it was running. I unplugged the wire from the map sensor and it started running close to normal. Weird. So i swapped the old map sensor out with a new one but no fix. I honestly dont know what it could be. Something to do with the computer im guessing. Maybe the pcm board? I heard these engines have problems with afm lifters but not sure what to think about that. My afm isnt working. It stays on 8 cyl all the time. Also command start doesnt work anymore.

 

Sorry for the novel but im kinda desperate for help. Thanks again

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You said that you checked the injector and that didn't help. But that was when you only had 0 to 30 psi. of compression. Have you went back and check the injector after the head job? I would check and see if each injector is firing. And then ohm out each injector and do a fuel pressure drop on each injector.

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Thanks i will check and or replace the injector. Afm stands for active fuel management. And when under light load will deactivate 4 cylinders. Correct me if im wrong here. Since the problem started it has never once dropped the 4 cylinders.

Thanks again

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  • 4 months later...

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