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Looking for help with high RPM miss

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  • Looking for help with high RPM miss

    My circle track car has developed a miss at high RPM. It only starts after 30 or 40 laps, so the car is good and warm. As soon as it cools down the miss goes away. Not that the temp is coming up. The guage stays right where it should. I run 2 fans and an over size rad. But toward the end of a 50 lapper I have to feather the throttle on the straights to stop it crapping out. I have replaced all the usual ignition parts (plugs wires rotor and cap)
    Last night I ran a 40 lap main and it missed for about the 5 last laps, and thanks to impending rain that didn't come, I went right back out 15 minutes later and did a 10 lap dash. It ran like a top for the dash.
    (3rd in the A main and 1st in the dash)

    Oh, and its a twin cam out of a 1992 in an older car. Its run all season without a hitch until 3 races ago. Emissions are all gone, but other than that its bone stock, including the timing.

    Any thoughts are appreciated.

    Thanks for the help

  • #2
    you might want to try running a compression test as soon as you come in, or some other time when its so hot it is missing.


    • #3
      What is your oil temp? Side note - the best plug to use for an NA KA is the Champion C57. Plus it's nice you don't adjust the gap. Leave it as is.
      NASA Time Trials TT3

      BTW I work for Garrett


      • #4
        Does it misfire only under load? Off the top of my head... If the misfire completely goes away when lifting off the throttle, then for one possibility, I would suspect an intermittent ignition coil failure, that only happens when hot. At higher rpms the coil is going to be saturated for a larger duty cylce, also the extra load is going to really push the ignition system to the max.

        One thing you could do is bring the car in to the pit's after the probelm is occuring, and load up the engine on the brakes & try to get it to misfire. Then disconnect a fuel injector plug or spark plug wire (if it's a dead miss) & figure out if it's one particular cylinder or if it's something related to all the cylinders.

        Another idea would be to ohm check the ignition coil right after you get off the track. My last idea for now, is that you could run the engine @ your garage/house & use a hair dryer to heat up one suspected component (or connection) at a time until you find the coulpret. One thing to remember about ohm checking parts is that if it check's out ok, it might be ok, but if it check's out to be way out of spec then it is bad. The meter supplies a very small amount of current to check resistance in a circuit which is not enough to find certain faults (like shorted windings in a coil).

        Something I have to always remind my service writer @ work is that, I need to be able to duplicate the problem while inspecting the vehicle, otherwise trying to track down an intermittent issue can be an absolute nightmare!


        • #5
          Thanks for the replies. I was suspecting the coil as well. The miss is only under load and I have not been able to recreate it in the pits. If I lift off the throttle a bit and then ease back into it, I can prevent it. And it seems like very little time has to elapse for it to cure itself. A few laps under yellow will clear it up for a while.


          • #6
            Signs seem to point to a tired ign coil.

            Honestly, my SR20DET would break up over about 16 psi of boost with copper plugs. Moving to iridiums 100% fixed it. They are trying to ionize a gas volume that's about 30% the size, so it just takes less potential to jump the gap.

            Might be worth giving a set a try since they only cost about $6-7/ea.
            '18 Chevrolet Volt - Electric fun hatch for DD duty!

            DefSport Koni Sleeve and Spring Perch Buy!!!


            • #7
              I logged on to update another thread, and noticed/remembered this one. For what its worth, it was the ECU. Go figure. I replaced half the electronics before trying that.