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Multiple cylinder misfire

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  • Multiple cylinder misfire

    I’m having a problem with multiple cylinder misfire‘s. I have change the power pack, wires, spark plugs, head gasket, intake gasket, PCV valves, EGR, I have checked the wiring connections, I have checked the injectors and wiring, no matter what I change as soon as I pull out of the driveway and get up to 20 miles Per hour I get the check engine light flashing and the code is multiple cylinder misfire! What else could I be missing?
    Live to Ride
    All mud all day if i could!!!!!!!

  • #2
    Sorry, for a delayed response on my part.

    I'll go basic, since you didn't list your year/engine.

    3 things to run:
    Air, Fuel, Ignition.

    You've pretty well hit up the ignition side of it, so Air and Fuel would be the next two concerns.

    Without a fuel pressure tester, you'll be hard pressed to get an accurate estimate of whether or not you're getting the fuel you need. But, if you're Jeep has lots of high miles, might be time to replace your fuel filter. Also, was this a sudden onset after a fillup? Might check your fuel for water.

    Ignition is pretty much a constant, and as long as your system voltage is good, you're rarely going to get a misfire that is ignition related and only rears its head at specific RPM. Most often RPM related misfires are either fuel starvation or an airflow restriction. The engine is in essence an air pump. If the intake is jacked or if the cats come apart or something you can get some issues.

    If the engine starts and idles pretty much normally, then falls on its face above specific RPM, I always suggest investigating the fuel system first.


    • #3
      2007 JK Unlimited

      I have Pulled the intake off and checked fuel pressure and injectors, I changed the air filter, even went back to factory plug after hearing that might be an issue. I have spent close to 3 months trouble shooting Replacing head gaskets, intake gaskets, checking possible wiring issues!! I have done leak down tests. Compression tests. Nothing pointing to a cause.

      My friend says try searching it like this "p0300" this was the top selection. It says to look at every thing I have done BUT it says you may have an issue with the catalytic converter!!!!!

      I took my exhaust manifolds off and found that my catalytic converter was completely disintegrated and crumbled down into the lower cat on the driver side!!! I attempted to see if there was any airflow through the exhaust pipe and found the pipes on the drivers side were completely restricted!!! NO airflow what so ever!!! I had the catalytic system fixed and not only is my Jeep running better than ever!! I no longer have an overheating issue that i have been chasing for the past 3 years. It appears the upper cat had started falling apart and slightly restricted the lower cat causing the drivers side to heat up causing my over heating issue. Over the past 3 years i have replaced thermostats, water pumps and put a high dollar radiator in the attempt more water and cooling would help the issue. The Jeep was running fine it was just running hot.

      The P-0300 code is the most evil code, The system sees there is an issue but cant pinpoint it in any way. I am hoping this help other that are chasing these types of issues.

      Good Luck and Happy Mudding!!!!!!
      Live to Ride
      All mud all day if i could!!!!!!!


      • #4
        glad you got it. that is relatively rare on these Jeeps. Yours is only the second I've heard of that developed a mechanically failed cat.

        Didn't notice an overheating remark in your post. Sorry bout that.

        My 3.8 had one, but it was most definitely the thermostat. An Advance Auto Parts replacement that lasted about 10,000 miles. pan tested it and it was only opening a very tiny amount. New one I tested next to it and it opened up three- four times as much. Ya never know.