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2012 Tahoe 5.3 with Misfire on 4 ...

Discussion in 'Engine & Drivetrain' started by ivan, Jan 19, 2021.

  1. wjburken

    wjburken Supporting Member

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    A quick question after rereading some of the posts here.

    How did the initial mechanic determine that the AFM lifters were fine?

    Personally, given all the issues that folks have had with AFM lifters going bad, I would have considered replacing the lifters when he had the head off to check them, but that’s just me. I mean, he was already there.

    That being said, I would check everything involved with #4. Start from square one. Swap plugs with #2 and see if it moves. Swap wires with #2, swap coils, etc. I still think there is value in checking you coil harness as well. There was a guy on here a while back with similar issues, but on the other side of the motor. He ran in circles doing everything except check the harness and that’s what it ended up being. When trouble shooting, you have to rule out stuff, not just dismiss stuff. For work and on my vehicles, I’ve had too many brand new parts that were DOA and too many seemingly unlikely things end up being the cause to not look at everything.
     
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  2. wsteele

    wsteele Full Access Member

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    I agree.

    Not knowing exactly what has actually been done in what exact sequence versus what is believed to have been done makes advice pretty iffy.

    Frankly I was surprised to read his mechanic friend pulled the head to check the lifters. I mean, I think most would be fine with a quick inspection of the rockers functioning on that bank to rule out a lifter collapse as the misfire cause. Also, he probably would have just heard it clapping away.

    The other problem is the complexity of how these systems work and how that might confuse perceived results. For instance, the P0300 is a permanent code. So if cleaning the MAF had actually addressed the multiple random misfire, a successful completion of the trip criteria so the ECU would clear that code likely would still be awaiting the ECU's decision.

    It kind of becomes a game of blind man's bluff. Probably very frustrating for the OP.
     
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  3. Leevon

    Leevon Full Access Member

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    Maybe he meant "pulled the head cover"? I think that's what GM calls them, the OP is Canadian...I'm reaching though.
     
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  4. wsteele

    wsteele Full Access Member

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    I think GM still calls them valve covers, but good point about the lost in translation effect. Could be a simple case of misnomer. Good/bad nomenclature definitely affects the Blind Man's Bluff quotient. :(
     
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  5. Leevon

    Leevon Full Access Member

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    Better yet they come up with this lol:

    upload_2021-1-22_11-55-57.png
     
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  6. ivan

    ivan TYF Newbie

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    Hello all,

    A compression test was done at the time and all good.

    I pulled #4 spark plug and it was black. And took some readings
     
  7. ivan

    ivan TYF Newbie

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    Some pics of reader

    442B7957-796E-4957-BA2C-D03CFC11A1CD.jpeg 90B41F52-B564-4BB6-B7A0-85D4B312A6FF.jpeg
     
  8. wsteele

    wsteele Full Access Member

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    Can you select injector pulse width on that unit in live data?
     
  9. wsteele

    wsteele Full Access Member

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    Also, if you find the time to dick with that scanner, see if it will display fuel trims.

    Are the upstream O2 sensors original?
     
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  10. Leevon

    Leevon Full Access Member

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    I just bought this 2007 Tahoe with 258,000 miles (see sig link). It had a misfire at idle, rough idle, and P0300, PO304 just like yours. Found severely fouled plugs, #4 was the worst. We replaced the plugs and did a top engine clean (intake clean). She's running like a champ now. I wonder how closely your friend examined the valves. Another possibility could be plugged cat which can cause PO300.
     
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