2000 Mazda Miata P0304 Misfire Diagnosis: Spark Plug, Injector or Mechanical Issue?

The P0304 code indicates cylinder 4 is not contributing properly to engine operation. Possible causes include: no spark (faulty ignition system), a bad spark plug or wire, a malfunctioning fuel injector (clogged, leaking, or non-pulsing), or mechanical issues such as low compression or vacuum leaks. Check if cylinder #4 is producing a visible spark. Observe the plug's color—black indicates rich mixture, tan suggests partial wear, and dry means lean conditions. To assess injector performance, perform a fuel pressure test and monitor leak-down rate at engine idle (KOEO). Always use OEM-quality parts; avoid aftermarket substitutes that may compromise reliability.

Thanks for the input, kev2. I initially replaced the plug wires because Miata forums suggest they typically last only 30–50k miles—my car has 74k and had never been serviced on this component. After replacing them, the misfiring stopped entirely and the engine runs smoothly now. All spark plugs appear in good condition with a healthy golden-brown color, and cylinder #4 is sparking visibly. However, I'm unsure how to test for injector pulsing or conduct a fuel pressure check. Also, I'm confused about why the engine runs fine yet still shows P0304—does this mean the code isn't reflecting real-time performance?

The presence of DTC P0304 means the ECU detects cylinder #4 is not performing its share of engine work. Common causes include ignition, fuel delivery, or mechanical faults. I noticed your spark plugs have only 17k miles—this suggests they're in good condition. Are you using NGK or Denso? Also, ensure the timing belt has been replaced (it's due at around 60–80k miles). Try adding one can of Techron or Seafoam fuel treatment to improve combustion efficiency. Avoid oiled air filters—they can interfere with engine performance. If symptoms persist, use an OBD-II scanner to monitor live data: fuel trim values (FT), ECT, MAP, MAF, misfire counter, and CKP sensor patterns. For detailed reference, see https://www.obd-codes.com/p0304. Note: A DTC does not always mean a failed part—sometimes it reflects an intermittent or performance issue.

Thanks again, kev2! The spark plugs are Denso KI6PR-U 11 with only 17k miles. The timing belt was replaced long ago. It's interesting that you mentioned Techron/Seafoam—just before the problem started, I filled up with gasoline. Could this have contributed to a buildup of contaminants affecting combustion?

If your car has been well-maintained and remains stock-spec, consider simple fixes first—like adding fuel treatment. Double-check the spark plug type (Denso or NGK) and confirm you're not using an oiled air filter. A thought: when visiting a parts store for fuel additive, ask them to scan your vehicle’s codes—many offer free OBD-II scans. Even if your scanner is basic (like the Autel MS300), their professional device might detect intermittent issues you miss. This could be a valuable step before investing in expensive diagnostics.

The spark plugs are Denso KI6PR-U 11 with only 17,000 miles. I pulled them out and they look in excellent condition. Thanks for the tip—when I go to pick up Seafoam, I’ll ask the parts store to scan the car. My Autel MS300 scanner costs just $14.49, so it’s likely not high-end or fully reliable. Thanks again for your help, Corkabias!

Hi kev2, I just replaced the spark plugs today and now the Check Engine Light is off. The engine runs smoothly with no misfires. Thank you so much for your detailed guidance—it made a big difference in diagnosing this issue. Best regards, Corkabias