97 GMC C1500 Misfire

This topic contains 3 replies, has 2 voices, and was last updated by Tom Tom 1 year, 3 months ago.

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  • #953813

    Hi all! Sorry for the wordy post. Short version is I have a #2 misfire and everything appears to be working properly, but suspect the thing is just worn out and needs rebuilding/replacing.

    1997 GMC Sierra C1500
    5.0 with 188k

    Quick history:
    Bought the truck needing intake manifold gaskets. While I was in there, it also got a MPFI upgrade and distributor as they were both trash. The motor was pretty sludgy under the valve covers and in the valley. Cleaned that up as best I could.

    The good:

    – Timing was set correctly with a scanner.
    – Vacuum gauge shows on the low side of good, and steady throughout RPM ranges.
    – Fuel pressure good and holding.
    – I’m assuming the injectors are operating correctly, since they’re under the intake and I can’t test them individually. I did remove the upper intake again to make sure I installed the spider correctly and didn’t see anything out of place.

    The bad:
    – I have both P0300 and P0302 codes. DashCommand only shows misfire counts on 2.
    – Compression on all cylinders is between 140-150 dry. Adding oil to #2 raised compression to 170, which seemed significant.
    – All plugs show some oil fouling but I don’t run the truck much, and have tried new plugs with no improvement.
    – Spark seems strong at the #2 plug (it shocked me) but pulling the plug wire while running shows no significant drop in idle, especially compared to other cylinders.
    – Fuel trims are all over the place

    Does that about cover it? Worn rings and/or heads? Can a mechanic do any additional scanning or diagnostics? Truck is in average condition for its age and I’m not sure it’s worth a crate motor, if it ultimately needs a new one.

    Thanks for reading. Your thoughts are appreciated.


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  • Author
  • #953814
    Nightflyr *
    Richard Kirshy

    You state vacuum om the low side of good and fuel trims all over the place.
    Might suggest smoking the engine and look for any vacuum leak(s)


    You’re probably right there. I haven’t smoked it but using propane I’ve found and fixed a couple vac leaks. And now that I’ve got plenty of time I’ll build a smoker and report back. Thanks!


    Well, fixed it, it was all my fault. Turns out I bent a pin on the fuel injector plug. Straightened the pin and carefully plugged it back in. No codes and runs great 😀

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