1997 Pontiac Sunfire Misfire Code P0300 - Fan Runs Early and Engine Stalls at High RPM
My 1997 Pontiac Sunfire is throwing up a misfire code P0300. I've already replaced the spark plugs and ignition wires, which were confirmed to be in good condition. The strange part is that as soon as I start the engine, the cooling fan activates—this fan only comes on when the engine reaches full operating temperature, usually after it's warmed up to near overheating levels. The engine runs erratically and will not rev past 3000 RPM. This issue has occurred twice before: first time, replacing the throttle position sensor (TPS) resolved it temporarily. Second time, cylinder 1 had a faulty plug—after replacement, the code cleared immediately. However, I'm now unable to identify the root cause of this recurring misfire. Any insight into possible causes or diagnostic steps would be greatly appreciated.
Make sure your OBD2 scanner reads all codes—especially P0999 and above. Did you use AC Delco spark plugs? Please check and share your fuel pressure reading. Is the fan running only when the AC or defrost is on, or does it activate independently? Also, confirm that your vehicle is stock with no aftermarket modifications.