#2 Misfire at Idle, Random/Multi While Driving

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Schurkey

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Cranking compression test to verify ring and valve sealing. Pull valve cover, assure that valves open 'n' close, and the valve springs aren't broken.

Assure cap, plug wire, and spark plug for #2 are in good condition--good insulation, plug not fouled, etc.

If it's not compression or ignition, it's fuel.
 

boy&hisdogs

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What about vac leak? I know the intake manifold gaskets on these trucks like to go bad. I had mine replaced over 100k ago.

Is there a good way to test that without a smoke machine?
 

stutaeng

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Per my post #2:
"It's pretty easy to do a good guess if it's a vacuum leak or something else. Rev up the engine and see what the fuel trims do."

If your LTFT is 25 at idle, but lowers (or improves) as you increase the RPM, then look for vacuum leaks. If fuel trims don't change much, then not likely vacuum leaks.

The reason is that the leak is a larger percentage of the metered air going inside the engine. As the engine RPM is increased (and so does metered air), the leaked air stays relatively the same, so the ratio is improved at higher RPM.
 

boy&hisdogs

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Per my post #2:
"It's pretty easy to do a good guess if it's a vacuum leak or something else. Rev up the engine and see what the fuel trims do."

If your LTFT is 25 at idle, but lowers (or improves) as you increase the RPM, then look for vacuum leaks. If fuel trims don't change much, then not likely vacuum leaks.

The reason is that the leak is a larger percentage of the metered air going inside the engine. As the engine RPM is increased (and so does metered air), the leaked air stays relatively the same, so the ratio is improved at higher RPM.

LTFTs get worse/higher the more I rev
 
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boy&hisdogs

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Well I drove it again and found something interesting. The LTFT doesn't correlate 100% with the RPMs but the STFT does much more closely. More throttle gives me less short term fuel trim, and chopping the throttle makes it spike.
 

boy&hisdogs

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Another development...

It's acting like it's vapor locking. It cold starts fine, but this morning I stopped to get gas and it wouldn't start up again. I had to let it sit for a while then when it did finally start it was a rough start. Once driving though it was back to normal. It's done this a few times now.
 

stutaeng

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It's weird sometimes on the fuel trim codes. A few weeks ago I was helping my nephew diagnose his 4.3 Vortec S10. The thing was running like crap. Had a hard time restarting when warm. One bank was running like 50 percent rich, but no rich codes!

We hooked up a fuel pressure gauge, and the pressure went from like 60 to almost zero in 30 seconds. Either an injector was stuck open or the FPR was leaking.

He got a new spider assembly (comes with a FRP) and it runs smooth again.

Another development...

It's acting like it's vapor locking. It cold starts fine, but this morning I stopped to get gas and it wouldn't start up again. I had to let it sit for a while then when it did finally start it was a rough start. Once driving though it was back to normal. It's done this a few times now.

Did you ever hook up a fuel pressure guage?
 
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