'07 Tahoe #4 Misfire

Discussion in 'General Tahoe/Yukon/Escalade Discussion Forum' started by mrfix71, Jul 22, 2019.

  1. mrfix71

    mrfix71 New Member

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    I am stumped. There was a misfire on #4 after pulling into town after a moderate road trip. The plug was fouled up and the electrodes were bridged with carbon. Ended up replacing all 8 with AC Delco iridium plugs and wire set. Still misfiring and code P0300 (random misfire) pops up repeatedly. Occasionally a misfire #4 will also appear as I test and clear codes. The scanner I have is a fairly basic model but has been a God-send when troubleshooting newer vehicles.

    I replaced the #4 coil after my strobe failed to light (it did on the others). While it initially lit up the strobe, the misfire never went away. I drove about 30 miles with a can of Seafoam mixed in a 1/4 tank to work and the engine misfired the whole time without improving.

    Tested when I got home and today there was no spark again on #4. The injectors all tick equally listening with a stethoscope. Fuel pressure was about 60psi (IIRC) and the gas that was sampled in a jar looked clean.

    Here is where I am really stumped...if I temporarily connect an old wire and spark plug (leaving the newer plug in the engine) I can get a strong spark, as well as swapping out the new plug and wire. However, if I properly install them in any combination in the engine, there will not be a spark indicated by the strobe. Never seen that before!

    Another tidbit is the cylinder compression (tested with fuel pump disabled) will peg well over 150 psi for the first cycle then drop down to about 30 for the next several piston strokes. If I wait a few seconds, the same results. Starts out strong but drops off. The neighboring cylinders (2&6) hold steady at about 150 each (IIRC).

    I am wondering if sticking valve or collapsed lifter causing the low compression. If the cylinder is simply carbon fouled, I cannot get a spark to burn it away while driving.

    Virtual 6-pack of favorite beer for anyone who can help with this!
     
    Last edited: Jul 23, 2019
  2. mrfix71

    mrfix71 New Member

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    Just took the passenger valve cover off and discovered #4 intake valve was flopping around. Looks like a collapsed lifter.
     
  3. mrfix71

    mrfix71 New Member

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    Just tried this trick and it worked to free the lifter. Going to bypass the fuel management system and leave it in V8 mode.
    https://www.youtube.com/watch?v=pP7I5tbXsfw

    Now to put it all back together. Hopefully there will be spark and Big Blue will be up and running again.
     
  4. mrfix71

    mrfix71 New Member

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    Got everything assembled but lost two injector clips (ping!). Tahoe started up and ran great but need to replace one of the injectors' seals.

    After all the expectation of having to tear the engine down and buy a cam/lifter kit...having to buy just the seals for an injector and a couple of clips is a relief. Gonna do an oil flush/change just in case.