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Rough Idle and Misfire Codes on 2018 BMW M3 (S55)

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Mike T. · rep 0 · May 25, 2:58 AM · 1 replies · solved · Question

Vehicle: 2018 BMW M3

My 2018 BMW M3 (S55 engine) has developed a rough idle and occasionally stutters under acceleration. The check engine light came on, and when I scanned it, I got P0301 (Cylinder 1 Misfire Detected) and P0300 (Random/Multiple Cylinder Misfire Detected). It feels like it's running on fewer cylinders sometimes.

I'm at around 50,000 miles. Could this be spark plugs, ignition coils, or something more serious like an injector? What's the best way to diagnose this without just throwing parts at it? I want to make sure I fix the right thing.

✓ Accepted solution

ClearTheCode Admin

ClearTheCode Team · rep 0 · May 25, 2:58 AM

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ClearTheCode admin answer:

Misfire codes (P0300, P0301, etc.) on an S55 engine at 50,000 miles are common and usually point to ignition system components or fuel delivery. It's crucial to diagnose systematically to avoid unnecessary parts replacement.

1. Common Causes of Misfires:

  • Spark Plugs: These are wear items and should be replaced according to the maintenance schedule (typically around 40-60k miles for performance engines). Worn plugs can lead to weak spark.
  • Ignition Coils: Coils can degrade over time, leading to intermittent or complete failure, causing a misfire on a specific cylinder.
  • Fuel Injectors: A faulty fuel injector (clogged or failing electrically) can cause a cylinder to run lean or rich, leading to a misfire.
  • Carbon Buildup: Direct injection engines like the S55 are prone to carbon buildup on intake valves, which can restrict airflow and cause misfires, especially at idle.

2. Diagnostics Steps:

  • Swap Components: Since you have P0301, swap the ignition coil from Cylinder 1 to a different cylinder (e.g., Cylinder 2). Clear codes and drive. If the misfire moves to Cylinder 2 (now P0302), the coil is faulty. If it stays on Cylinder 1, swap the spark plug from Cylinder 1 to another cylinder. Repeat the process. If the misfire still stays on Cylinder 1, it points towards the fuel injector or carbon buildup.
  • Check Fuel Injector Pulse: A diagnostic tool can monitor injector pulse width and fuel trims to identify issues.
  • Compression Test/Leak-Down Test: If ignition and fuel are ruled out, these tests can identify mechanical issues like valve problems or head gasket leaks, though less likely for just one cylinder unless severe.

3. When to See a Shop:

  • If swapping coils and plugs doesn't resolve the issue.
  • If you suspect a faulty fuel injector, as replacement requires special tools and coding.
  • If you suspect carbon buildup, a walnut blasting service may be needed.
  • If compression or leak-down tests are required, or if the issue is intermittent and difficult to pinpoint.
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