2000 Mazda Miata P0300 Random Misfire Diagnosis and Fixes
First time posting — though I've been working on cars for 40 years, the P0300 random misfire code on my daughter's 2000 Mazda Miata remains challenging to diagnose. The car runs roughly after it warms up. So far, I've replaced the PCV valve, vacuum lines, and cleaned the crankshaft position sensor. I'm concerned about wasting money on unnecessary part replacements. Can anyone offer practical troubleshooting steps or common causes for this issue?
Check the spark plugs — make sure you know which cylinder each one came from. Look for signs of wet, dry, black, or tan deposits. If replacing them, use OEM-quality plugs. Perform a fuel pressure test and conduct a fuel pressure leak-down test to verify the fuel pressure regulator is functioning properly. There should be no fuel in the vacuum line. Use an old-school spray method to check for vacuum leaks. Also confirm that the ECT (engine coolant temperature) sensor is working — ensure your temperature gauge reads accurately. Ideally, use an OBD2 scanner to view real-time engine temperature data.