Rough Idle & Misfires on 2019 M2 Competition (F87)

Misfire codes (P030x) on an S55 engine at 45,000 miles are not uncommon and typically point to ignition system components. Given you have multiple misfires, here’s a systematic approach to diagnose and resolve the issue:

1. Spark Plugs:

   • Inspection/Replacement: At 45,000 miles, your spark plugs are likely due for replacement, especially on a high-performance engine like the S55. Worn plugs are a primary cause of misfires. Inspect them for fouling, wear, or incorrect gap.
   • Recommendation: Start by replacing all spark plugs with OEM-specified plugs (e.g., NGK SILZKBR8D8S). Ensure they are properly gapped if not pre-gapped, and torqued correctly.

2. Ignition Coils:

   • Diagnosis: If new spark plugs don't resolve the issue, the next most likely culprit is the ignition coils. Since you have misfires on cylinders 1, 2, and 3, you can try swapping the coils from the misfiring cylinders to known good cylinders (e.g., 4, 5, 6). If the misfire codes follow the coils, then those coils are faulty.
   • Recommendation: Replace any faulty coils. It's often recommended to replace them in sets (e.g., all 6) for consistency, especially if they are original and at this mileage.

3. Fuel Injectors:

   • Diagnosis: While less common than plugs or coils, a failing fuel injector can also cause misfires. This is usually indicated by a specific injector code (e.g., P020x) or if misfires persist after replacing plugs and coils. A fuel system cleaner might help if it's a minor clog, but a failing injector typically requires replacement.
   • Symptoms: Besides misfires, a failing injector might cause a strong fuel smell, rough idle, or poor fuel economy.

4. Other Potential Causes (Less Common):

   • Vacuum Leaks: Can cause a lean condition and misfires.
   • Compression Issues: A compression test can rule out internal engine problems.
   • ECU Issues: Very rare, but possible.

Common Causes for P030x Codes:

• Worn or fouled spark plugs.
• Failing ignition coils.
• Faulty fuel injectors.
• Vacuum leaks.

When to See a Shop: If replacing spark plugs and diagnosing/replacing ignition coils doesn't resolve the misfires, or if you suspect a fuel injector issue, it's best to consult a BMW specialist. They have diagnostic tools to perform more in-depth tests, such as fuel pressure tests, injector flow tests, and compression tests, to accurately pinpoint the root cause without guesswork.