misfiring issues, need help

badassmommy2015 · August 10, 2015

Im no mechanic to begin but i have done plenty of research and done enough service on my vehicle to know a little something. I have also had a certified mechanic of 40 years under the hood with me today and he left stumped as well. I need help from anyone who has any suggestions!!! I really am trying to avoid taking it to the dealership, that only creates more issues in my opinion. The people in the service department wont give any information just tell me to bring it in and pay a shit load to have them do it and i cant afford unnecessary repairs. with that said this is whats going on and the process in which i have gone with.

I have an 09 silverado 1500 5.3L V8 325 flex fuel with 102,xxx miles on it. I have been getting codes for some time now for a misfire on cylinder 8 (P0308). starting with the obvious i replaced the spark plug but it didnt fix the misfire, the idle is also very rough. When the truck is idling is has a quiver and a shake obviously from the one cylinder not working properly. I occasionally get an EVAP code but thats only sometimes. Having the issue resolved down to that one cylinder i went about replacing the spark plugs twice, the spark plug wires, swapped coil packs, fuel injector, as well as the air filter. i tested all wires and hoses for issues and nothing. I unplugged the mass airflow sensor and thats not it either. When i start the truck i can hear a faint pinging sound underneath the passager front wheel well that sounds like a pain in my ass. The noise kind of sounds like the piston is hitting the valve. I just added some seafoam to the gas tank and im about to drive it around and get it circulated as well as run a compression test in the morning. If anyone has a similar issue i would love to hear about your experience and what helped you. Today i spent hours out there in the hot f-ing sun taking off the fuel rail, throttle body and intake basically the whole top of the engine to remove the tip to one of the fuel injectors that fell down in there before it caused major problems. How it got stuck in there im not sure not like those things just fall off, after removing the fuel rail i noticed that the tip was gone and couldnt be seen and i wasnt comfortable just leaving it like that so after breaking it down i found the 2 pieces sitting right on top of the closed valve. Now i bought this truck preowned with 85,000 miles on it and it wasnt long before i got this issue. The mechanic i have helpin me is stumped, he cant think of anything else that would cause a misfire on that one cylinder alone. the misfire is not random its always on cylinder 8. I have the truck covered under factory warranty for another 6,000 miles and i will take it in if compression is low in the morning and extensive repairs are needed on the engine but if its something i can fix for a decent price and my own hands i would rather. i dont trust shops all they do is try to create more issues and make you believe you need more work than necessary, I really dont want to hear all that bullshit.

i havent checked everything i wanted to check yet but this is my list, i figured i would get a post going to see which are most suggested to test first

leaky head gasket
bad o2 sensor
cam shaft sensor
DME or ECU
intake manifold gasket leak
Idle control valve
throttle position sensor
fuel pump
cam lobe
cam lifters
pressure regulator
mass airflow sensor

I will test until i find the issue for anything i am able and after that if i havent found it i guess i have to take it to the dealership. anybody anything????

BlackZ71Silverado · August 12, 2015

Quit throwing parts at the truck. That's not going to do anything but lighten your wallet and frustrate you more because your problem did not go away after spending all of that money.

You should do a compression test for kicks but I doubt it's going to tell you anything other than cylinder #8 is not functioning properly.

Are the fuel injectors for that cylinder clean and spraying properly?

Since it's a constant misfire on that same cylinder every time, I would guess you may actually have bad valves/rings on cylinder #8.

It sounds like the piston rings or valves, or both may be shot on that cylinder. That would not allow that cylinder to get proper combustion of the fuel in cylinder #8. That will definitely cause a misfire like you describe and will also cause the rough idle.

Does your truck use oil? Do you have to add any oil between changes?....Don't know what else to do, you're going to have to take it into a good shop that you trust can do the work properly.

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Badass69 · August 12, 2015

Cylinder 8 doesn't have an afm intake lifter in it but depending on the compression test you could look at lifter failure. The supposed "LS7" lifts gm uses aren't very good... and an 09 is amidst the worst of them. Something else to consider. Could always be the injector on that cylinder as well.

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