Q: 1996 corvette random misfire at highway speeds

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1996 corvette LT1 138k miles auto trans. P0300 random misfire code. Check engine only comes on at speeds 60 mph and above and stays on/ blinks. Have replaced optispark with MSD distrubtor and wires, plugs also replaced. New fuel filter, replaced vacuum lines. Compression tests all 120+. Replaced PCV and EGR and Purge. P0300 continues to pop back up on the dash after about 3 days of driving. Car is straightpiped but not sure if that would have anything to do with it. Misfire counter showed zeros across board when at idle or parked with throttle at high rpm. Just when it gets on road at consistently 60 mph the check engine comes on.

A possibility is vacuum leaks around the intake. If the lobes on the camshaft are worn, that can cause a misfire and other inefficiencies as well. I would first check for a vacuum leak. If no leak, secondary firing patterns on all the cylinders should be examined on an oscilloscope. The waveform will help narrow the potential causes to fuel versus ignition. Another possibility is the random misfire is due to intermittent faults in the injector and/or ignition components so although these components appear to be in spec during static testing, in fact they are not reliable and are later, in service, operating intermittently. Finally, the ignition and fuel driver circuits in the PCM for the misfiring cylinder(s) could be faulty; that is the absolute last thing to test though unless you happen to have handy a known good PCM that you can substitute. If none of these possibilities pan out, you can request a misfiring diagnostic and the responding certified mechanic will get this resolved. If you have further questions or concerns, do not hesitate to re-contact YourMechanic as we are always here to help you.

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