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Q: My car misfires at light throttle and mid RPM conditions

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What sort of failure would cause an engine to misfire at light to constant throttle at mid RPMs (i.e. midway between idle and red-line) It only occurs at part throttle at a very specific RPM and only after the engine has fully warmed up, about 5 minutes. It does not misfire when cold or during warm up and it does not misfire at larger throttle openings or at smally throttle openings at higher or lower RPM. Oxygen, throttle position, MAP, coolant temperature, crankshaft position and camshaft position sensors have all been replaced in spite of the fact there have been no CELs of codes stored in the ECU. Fuel pump, fuel pressure regulator and fuel filter have also been replaced. Swapping ECUs with a known good one has also been tried. Wiring harness and grounding points all inspected and tested. Various relays for fuel cut and other function have been replaced. Alternator has been rebuilt.

Testing with an oscilloscope shows coils and injectors cutting out at the time of the missfire

A: Without having the specifics on the vehicle...

Without having the specifics on the vehicle and engine type, I can only give you a general direction to take. You may have an EGR system fault. EGR - Exhaust Gas Recirculation - is designed to prevent knock, ping, and misfire under light throttle loads to help keep the combustion chambers cool. Since the exhaust gases have already been burned, they are cooler than the combustion chamber during ignition.

This prevents pre-ignition misfiring and it does not function at full throttle. A stuck/ inoperative EGR valve can contribute to the problem. Also, if any EGR tubes or passages are clogged with carbon due to age and mileage, the EGR "loop" will not function. I recommend having a certified mechanic, such as one from YourMechanic, come to your location to determine if the EGR valve needs to be replaced.

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