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  • Engine Misfire P0300

    1996 GMC K1500 XC 5.7L SFI

    My truck is misfiring. It is giving the code of P0300, which in my Haynes book says "Engine Misfire". It is running really rough consistent with a misfire. I've driven it around town and it is really down on power. I can't figure out what could be the problem. The motor is shaking pretty bad a certain RPM's. I have dual exhaust that is completely separate on both sides. One side of the exhaust is throbbing a lot more than the other side. Can't hear or see anything else weird in the motor.

    OK, so I've replaced:

    1. Both O2 sensors (pre-cat)
    2. Camshaft Position sensor
    3. Crankshaft Position sensor
    4. Spark Plugs
    5. Plug Wires
    6. Cap and Rotor
    7. Fuel Filter
    8. Cleaned MAF sensor
    9. Ignition Coil

    I don't know what there is left to do... I've done everything I can think of.

    Is there anything else that could be causing a misfire??? Or I could try changing????
    "A dollar saved is a DOLLAR"

  • #2
    Re: Engine Misfire P0300

    Originally posted by GMSEE
    1996 GMC K1500 XC 5.7L SFI

    My truck is misfiring. It is giving the code of P0300, which in my Haynes book says "Engine Misfire". It is running really rough consistent with a misfire. I've driven it around town and it is really down on power. I can't figure out what could be the problem. The motor is shaking pretty bad a certain RPM's. I have dual exhaust that is completely separate on both sides. One side of the exhaust is throbbing a lot more than the other side. Can't hear or see anything else weird in the motor.

    OK, so I've replaced:

    1. Both O2 sensors (pre-cat)
    2. Camshaft Position sensor
    3. Crankshaft Position sensor
    4. Spark Plugs
    5. Plug Wires
    6. Cap and Rotor
    7. Fuel Filter
    8. Cleaned MAF sensor
    9. Ignition Coil

    I don't know what there is left to do... I've done everything I can think of.

    Is there anything else that could be causing a misfire??? Or I could try changing????
    common causes of P0300 in the L31 motors could be the injector spider problems, fuel pressure regulator leaking, and intake manifold gasket leak. good luck, these radom misfires can be a pain to track down sometimes.

    EDIt- forgot, the dizzy gear can get worn in these trucks and cause the p0300 also.
    Last edited by Aloicious; 04-04-2009, 11:19 PM.

    1996 C1500 ECSB 5.7L AKA the "LS31", distributorless w/LS2 coils, RHS Heads, ZZ4 Cam, T56 6 speed, 12200411 PCM, 4.88s...Coming soon: forged twin turbo 383..

    Comment


    • #3
      Re: Engine Misfire P0300

      does it still have cats? im assuming it doesent but you know where that can get you.
      If you think health care is expensive now, just wait till its free!

      Comment


      • #4
        Re: Engine Misfire P0300

        Yes still has cats. It has two separate cats. What is the spider gear?
        "A dollar saved is a DOLLAR"

        Comment


        • #5
          Re: Engine Misfire P0300

          Originally posted by GMSEE
          Yes still has cats. It has two separate cats. What is the spider gear?
          spider gear? no, the dizzy gear is the gear on the bottom of the distributor that mates with the cam, the injector spider is the fuel injecting aparatus on the L31 motors, it consists of a fuel metering body and 8 poppet valves to act as fuel injectors, it looks like a spider is why its named that.

          here's some posts to help:
          http://www.fullsizechevy.com/forums/...m-misfire.html

          http://www.fullsizechevy.com/forums/...-misfires.html
          Last edited by Aloicious; 04-05-2009, 12:51 AM.

          1996 C1500 ECSB 5.7L AKA the "LS31", distributorless w/LS2 coils, RHS Heads, ZZ4 Cam, T56 6 speed, 12200411 PCM, 4.88s...Coming soon: forged twin turbo 383..

          Comment


          • #6
            Re: Engine Misfire P0300

            OK, thanks for the info. Does anyone know about the MAP sensor?Would this be causing a misfire? The Haynes book says the MAP sensor uses intake pressure to tell the PCM how to control fuel delivery and ignition timing.

            I've already replaced everything else that has to do with ignition timing.

            One thing to mention is when I put a spark sensor (non-invasive) on my wires and cap it only shows a spark every so often and very very random. Sometimes I won't see a spark for up to 10 seconds . It's completely random on every wire. When I put it to the coil wire it shows a constant light. Why would the spark to the plugs be so random?
            "A dollar saved is a DOLLAR"

            Comment


            • #7
              Re: Engine Misfire P0300

              Originally posted by GMSEE
              OK, thanks for the info. Does anyone know about the MAP sensor?Would this be causing a misfire? The Haynes book says the MAP sensor uses intake pressure to tell the PCM how to control fuel delivery and ignition timing.

              I've already replaced everything else that has to do with ignition timing.

              One thing to mention is when I put a spark sensor (non-invasive) on my wires and cap it only shows a spark every so often and very very random. Sometimes I won't see a spark for up to 10 seconds . It's completely random on every wire. When I put it to the coil wire it shows a constant light. Why would the spark to the plugs be so random?
              the MAP sensor is the "Manifold Absolute Pressure" sensor, it tells the PCM what the vacuum is in the manifold so the PCM can calculate the fueling. its possible that it could cause a misfire, but pretty unlikely. I'd really bet it is the distributor gear, that is a very common part on our trucks that will cause a P0300.

              I don't know abou the spark light, I would manually pull a few (one at a time) plugs, ground them and check for spark while someone cranks the engine over, if they were firing in 10 second intervals, the truck wouldn't even run. however, the goofy readings do suggest a spark problem over a fuel issue. which again, would point to the distributor gear being worn or a cap/rotor issue since the readings at the coil are consistant.

              does the cap bolt down tightly? there is a high likelyhood that the screws that hold the cap down have stripped or broken the plastic tabs on the distributor that they screw into, I had that happen to me and it wouldn't let the truck run at all. I have since put in a aluminum distributor to stop that from happening again.

              1996 C1500 ECSB 5.7L AKA the "LS31", distributorless w/LS2 coils, RHS Heads, ZZ4 Cam, T56 6 speed, 12200411 PCM, 4.88s...Coming soon: forged twin turbo 383..

              Comment


              • #8
                Re: Engine Misfire P0300

                the way you described the exhaust would make me think about possibly a clogged cat but idk.
                If you think health care is expensive now, just wait till its free!

                Comment


                • #9
                  Re: Engine Misfire P0300

                  on my motor the timing is set by the computer, so if I take the distributer out to replace the gear, how do I know how to put it back in correctly? Is it just by making a mark on the housing of the distributer where it meets the motor?
                  "A dollar saved is a DOLLAR"

                  Comment


                  • #10
                    Re: Engine Misfire P0300

                    Mark iot all before you take it out. If the gear needs changing, do so and then re-install exactly as you removed it. I always take a heap of pic's of everything I do....handy reference!
                    You may need to get the CMP retard set by someone with a scanner once you replace the dizzy gear.
                    Howard

                    YOU ONLY NEED TWO TOOLS IN LIFE - WD-40 AND DUCT TAPE. IF IT DOESN'T
                    MOVE AND SHOULD, USE THE WD-40. IF IT SHOULDN'T MOVE AND DOES, USE THE
                    DUCT TAPE.

                    98 K1500 Suburban, 4L80E, Whipple SC, Marine Intake, Hydroboost, LS1 0411 PCM.

                    Comment


                    • #11
                      Re: Engine Misfire P0300

                      Originally posted by Da Burb
                      Mark iot all before you take it out. If the gear needs changing, do so and then re-install exactly as you removed it. I always take a heap of pic's of everything I do....handy reference!
                      You may need to get the CMP retard set by someone with a scanner once you replace the dizzy gear.
                      yup, mark the dizzy before you pull it, that way it will run when you put it back in, I would take it somewhere that has a scanner able to read the CMP retard and have the timing set, shouldn't cost much, even at the dealer I would think it would be a pretty routine thing.

                      1996 C1500 ECSB 5.7L AKA the "LS31", distributorless w/LS2 coils, RHS Heads, ZZ4 Cam, T56 6 speed, 12200411 PCM, 4.88s...Coming soon: forged twin turbo 383..

                      Comment


                      • #12
                        Re: Engine Misfire P0300

                        If I take it out and replace it exactly the same way why then would I have to have the timing reset?
                        "A dollar saved is a DOLLAR"

                        Comment


                        • #13
                          Re: Engine Misfire P0300

                          Originally posted by GMSEE
                          If I take it out and replace it exactly the same way why then would I have to have the timing reset?
                          it would run but the timing on these is VERY sensitive (VERY we're talking a couple degrees could be like 1/16 of an inch turn), when I pulled my dizzy last time, I put it back in the same as when I took it out, and in reading the CMP retard with my efilive, it was still 14* off. you may have it close when re-stabbing it but chances are its not super precise, best just to get it set. the CMP retard needs to be at 0* +- 2*. you can definantly get it close so that it will run just fine w/o codes, but I'm too OCD to not have it set correctly on my truck, Howard will agree with that I believe. :aniteef: .

                          1996 C1500 ECSB 5.7L AKA the "LS31", distributorless w/LS2 coils, RHS Heads, ZZ4 Cam, T56 6 speed, 12200411 PCM, 4.88s...Coming soon: forged twin turbo 383..

                          Comment


                          • #14
                            Re: Engine Misfire P0300

                            Agree 100%. The damned things are so sensitive that just tightening the hold down clamp can throw them out a few degrees.
                            Howard

                            YOU ONLY NEED TWO TOOLS IN LIFE - WD-40 AND DUCT TAPE. IF IT DOESN'T
                            MOVE AND SHOULD, USE THE WD-40. IF IT SHOULDN'T MOVE AND DOES, USE THE
                            DUCT TAPE.

                            98 K1500 Suburban, 4L80E, Whipple SC, Marine Intake, Hydroboost, LS1 0411 PCM.

                            Comment


                            • #15
                              Re: Engine Misfire P0300

                              cool, I'll let you know how it goes. Thanks for the help.
                              "A dollar saved is a DOLLAR"

                              Comment

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