P0300 Misfire Diagnosis on 1996 Chevrolet Corsica After 3400 Engine Swap
I recently performed an engine swap, installing a 3400 engine into my 1996 Chevrolet Corsica, which originally had a 3100 engine. The vehicle previously experienced a cylinder 6 misfire due to a faulty head, which I repaired and has been running smoothly since. However, I am now receiving a P0300 diagnostic trouble code indicating multiple cylinder misfires. The car runs fine under normal driving conditions, but when idling down the block or at very low speeds (under 10 mph), it exhibits slight chugging and stumbling. I've replaced the catalytic converter with a new one and installed an EGR valve from a 2001 model. All components—including ignition coils, spark plugs, and wiring—have been recently upgraded and are brand new. Despite these updates, the P0300 code persists. Given that no other specific misfire codes appear, I'm seeking advice on how to accurately diagnose which cylinders are involved in the misfiring and what underlying causes might be responsible.
Check which specific cylinders are misfiring using a scan tool with live misfire monitoring or perform a cylinder cutout test. This will help identify whether the issue is isolated to one cylinder or affects multiple ones.