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At wits end - random misfire


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symptoms are:

 

Occasional misfire at idle (believe it’s after it goes into closed loop, because it runs fine for the first 30 seconds when it’s cold)

 

once a month I get p0171 and p0174 codes

 

Runs a little rough from idle to about 1/4 throttle then runs ok

 

No noises and runs great at greater than 1/4 throttle 

 

reports approximately 5 occurrences of misfire on cylinder 1 and 35 on cylinder 5 every week

 

I’ve replaced with new oem parts:


In tank pump

high pressure pump

injectors

manifold gaskets

mass air sensor

manifold pressure sensor 

temp sensor

knock sensor

o2 sensors

coils

wires

plugs

throttle body and position switch

 

my only idea is to now tear down the engine and look for bad valve springs /lifters

 

i’d really appreciate any ideas or comments before I do that

 

 

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What did the back of the intake valves look like when you replaced the manifold gaskets?  Did you find any oil in the intake manifold or the intake tubing?  I take it you did not replace all those parts at the same time, how many tanks of fuel have you gone through?  Do you always get fuel at the same place?  Any chance you have bad or watered gas?

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You replaced all those parts and it still hasn't fixed it? Wow.

 

Before I did another thing I'd do a compression test on the effected cylinders. I'd do a dry test, then a wet test. And if the doesn't show anything I'd trad it in. Screw that, what are you going to do? Replace every damn part of the truck? You're halfway there now. Get rid of it and don't buy another GM. 

 

I've got to hand it to you. I wouldn't be nearly as patient and understanding. I would never have gone through all the trouble of replacing all those parts. 

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if you replaced all ignition components , you might want to data log your fuel system before throwing money at it in parts... there is a filter on the high pressure side that I've heard can get dirty and might lean you out if it's plugged

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