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  1. #1

    Default 5.7 Vortec P0300 Random Misfire


    I am new to this site and desperate for assistance on a 99 Suburban 5.7 Vortec. Hopefully I can leverage community experience to resolve an issue that has me absolutely baffled!

    I purchased this vehicle a few weeks back to pull my boat. The SUV had a seized intake valve and corresponding bent pushrod on cylinder #4. I figured easy repair. I purchased a used head and assembled (with new plugs and gaskets on bank 2) within a few hours.

    The vehicle started right up and ran horribly. This is where the chase began. Following is a simple list of facts not diluted with a specific order of events, reasoning, or emotions!
    • Compression is great and consistent on all cylinders
    • Vacuum holds steady at 21”
    • Fuel pressure:
      • 62 psi - static
      • 54 psi – idle
      • 58-62 – under load while driving
    • Replaced the following:
      • Distributor cap, rotor, wires
      • Fuel Injector spider system
      • Intake gaskets more than once with the first repair and then again with the spider injector replacement
      • Ignition module
    • Installed distributor to spec after intake replacement. The first start after distributor installation triggered cam retard sync - CIL. I adjusted distributor a couple degrees and eliminated CIL. I had the same misfire prior to removing the distributor so doubtful it’s the issue.
    • Distributor drive gear looks perfect and there’s no slop in the installed unit.
    • Although vacuum test did not make me believe there was an exhaust issue, I removed the O2 sensors to help determine if this could be an issue of backpressure. I understand that the O2 ports are small but I’m confident that these ports in combination with what’s going through the exhaust (even if partially blocked) would allow it to idle.
    • I blocked off the EGR to no avail.
    • The issue gets worse as the vehicle warms. If goes from very bad to absolutely horrible.

    In summary the vehicle is misfiring all around. I can pull each plug wire individually and all plugs have some effect so we are firing on all cylinders. The entire vehicle shakes and is not drivable. It was obviously driven before the #4 cylinder head component failure that I purchase it with. I was certain it had to be related to the head since that was the repair that was performed. Since bank #2 has great compression and the engine has great vacuum I can’t see what the issue would be with the new head. I have no codes other than random misfire. I’m considering replacing:
    • Entire distributor (with new cam sensor)
    • Crank sensor??
    • Knock sensor??
    • Engine??
    • Vehicle???!!

    I have assumed that the O2 sensors are not the issue since the vehicle doesn’t run well in open loop either. I have laptop/OBD software that is not providing any valuable information in relation to this.

    I have read a lot of forums and many do not have closure or resolution. The forums with resolutions have mostly been that the spider system solved their issues but there were more that didn’t get resolved with spider. I’m sure I left something out but hopefully one of you patient guys that actually read all of this will come up with something that I have not considered! It will be a happy day when this is resolved…Thanks so much!


  2. #2


    It sounds to me like its a valve issuse. Probably not adjusted right and when it warms up and expands is throws it off a tiny bit more which makes it run worse. Did you make sure that the head casting number will work with your block? Sometimes even if its the same year the block and heads can be a little different in the valves and rockers. Also I have seen sometimes when pressure is removed from the lifters like when you take a valve pushrod out or losen the rockers it can cause very worn or damaged lifters to colapase or fall apart. It sounds more to me like its a valve problem though. I would think its even more of a valve problem because it gets worse as it heats up. Make sure that its all adjusted right and timed as well. I belive that you can adjust them with that bolt in the center of the rocker you can actraly do this when its running with the valve cover off. There is a certain amount of turns that there supposed to have when you put them on so make sure that you tightened them correctly as well. Check a repair manual for the specs or call a dealer.

    Hope this helps a little good luck!
    1995 C1500 4.3L 2WD. JET TBI Spacer, JET stage 1 chip, 10mm Taylor custom wires, Pulstar Iridium Pulse Plugs.
    260k miles and still going strong...

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