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Thread: Please help with weird misfire

  1. #1
    Tuner in Training
    Join Date
    Nov 2018

    Please help with weird misfire

    Hi guys, I'm trying to help my dad fix his 07 Tahoe, bone stock 5.3 flex fuel. We have a cylinder 5 misfire and we can not figure it out. I'm using HPTuners to scan and can see the misfire counter climbing steady and feel the misfire at idle. Sets Random misfire & #5 misfire codes along with both banks lean but the lean codes have not come back. Also says service traction control system but that's probably unrelated and more of a coincidence? I found a little corrosion on the grounds by the thermostat, cleaned that and no change. All fuses are good.

    So far we've swapped spark plug, wire, coil, and injector to another cylinder and the misfire stays on #5 no matter what. We took the valve cover off and all rockers are moving like they should. Checked for vacuum leak or exhaust leak but found nothing.

    One thing I noticed is that bank 1 02 sensor voltage is very erratic compared to bank 2. There are a lot more spikes in voltage on bank 1 instead of a nice swooping motion like on bank 2. I think the weird O2 readings are due to the misfire though, I can't see a bad O2 sensor causing only one cylinder misfire. I can confirm #5 misfire by unplugging the coil and nothing changes. Also, its reading 19.x% ethanol and I know it has 10%(my camaro has an actual ethanol sensor). This being a virtual flex fuel system, its probably due to the fuel trims being off because of the misfire.

    Another very weird thing is that I can go to the scanner special functions to turn off each injector one by one and I can turn off lets say injector 8 but I'll see the cylinder #1 misfire counter go up This happens on every injector, none of them correspond with the misfire counter. I checked the wiring diagram and injectors seem to be wired correctly. I'm assuming the misfire detection per cylinder isn't very accurate?

    My next step is to buy a noid light kit to confirm that the injector is getting signal from the ECU.

    Any ideas on what this could be? Dads Tahoe.jpg

  2. #2
    Tuner in Training
    Join Date
    Aug 2019
    What does the plug in number 5 look like? Any difference between it and the rest? On high mileage motors, valve seals can leak and let in oil, causing misfire?