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Random misfire that comes and goes

This is a discussion on Random misfire that comes and goes within the General Help forums, part of the LSx Technical Help Section category; ...

  1. #1
    Junior Member
    Join Date
    Jul 2021
    Location
    Macon, GA
    Posts
    2

    Pewter
    2000 Corvette

    Random misfire that comes and goes

    I am new to this forum and hopeful I can find an answer here. I have a 2000 C5 Corvette with 40,000 miles. A4 automatic trans, cold air induction, LS6 intake manifold, ported/polished throttle body, shorty headers, and Corsa exhaust. New plugs and plug wires.

    About 6 months ago it developed a random skip and threw a P0300 (misfire detected) code. The skip will come and go and will move from cylinder to cylinder. Appears at idle or off idle., clears up and runs fine. Then it will come back.

    Here is what I’ve done:

    Full throttle crank test (smooth as silk), checked coils (good), new plugs and wires, double checked plugs for gaps and cracks, and checked for vacuum leaks with brake cleaner (none).
    I also ran a series of diagnostic tests using GM Tech2 scanner.
    .
    DICM fault code P0300 (misfire detected).

    Fuel injector balance test: All 8 Cylinders pegged at 60psi then settled at 57 psi. #1 Purged at 47psi and all others purged at 46psi.

    MAF: 7.6g/s to 7.2g/s.

    Fuel Trims at idle:
    Short term - bank 1 (0) bank 2 (0)
    Long term - bank 1 (0) bank 2 (0)

    Fuel Trims at 3000rpm
    Short term - bank 1(0) bank 2 (0)
    Long term - bank 1 (-1) bank 2 (-1)

    All four O2 sensors tested good and wiring is clear and away from the headers.

    Misfire history
    #1 – 0
    #2 – 235
    #3 – 117
    #4 – 178
    #5 – 552
    #6 – 2
    #7 – 24
    #8 – 0

    Air/fuel ratio: 14.6:1

    Compression test all cylinders good.
    Leakdown test showed all cylinders good. C
    The mystery with this thing is the skip will move from cylinder to cylinder but mostly #’s 2,3,4,&5.

    Any ideas where to check next?

  2. #2
    Junior Member
    Join Date
    Sep 2021
    Location
    Georgia
    Posts
    4

    Black
    2001 Suburban

    Had this same problem and was associated with the coolant temp being too low at cruise. Im still working on it. Changing the thermostat helped, but still can not get the temp numbers to a good place.
    My cruise temp with the factory 187 tstat was only 154. I am by no means an expert this is my first LS. Just sharing my experience. P.S. Did you do a crank relearn?

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