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Rough Idle and Check Engine Light on 2018 Cooper S - Misfire?

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Steve N. · rep 0 · May 22, 6:54 AM · 1 replies · solved · Question

Vehicle: 2018 Mini Cooper S

Hey everyone,

I'm experiencing a really frustrating issue with my 2018 Mini Cooper S (F56, 2.0L turbo). Lately, it's developed a rough idle, especially when cold, and sometimes even at stoplights. The engine feels like it's shaking more than usual.

Yesterday, the check engine light came on. I pulled into an AutoZone to get the code read, and it came back as P0302, indicating a misfire on cylinder 2. I'm not noticing any significant loss of power while driving, but the rough idle is definitely concerning.

Has anyone else with a similar Mini experienced this? What was the fix? I'm hoping it's something relatively simple and not a major engine problem. Any advice on what to check first would be greatly appreciated!

✓ Accepted solution

ClearTheCode Admin

ClearTheCode Team · rep 0 · May 22, 6:54 AM

0

ClearTheCode admin answer:

It sounds like you're dealing with a common misfire issue, Sarah. A P0302 code specifically points to a misfire in cylinder 2. Here's a breakdown of potential causes and what steps you can take:

Common Causes for P0302 Misfire:

  • Faulty Spark Plug: The most common culprit. Spark plugs wear out over time, leading to inefficient combustion.
  • Bad Ignition Coil: Each cylinder has its own ignition coil. If the coil for cylinder 2 is failing, it won't deliver enough spark.
  • Clogged or Faulty Fuel Injector: An injector that isn't delivering the correct amount of fuel (either too much or too little) can cause a misfire.
  • Vacuum Leak: Leaks in the intake system can cause unmetered air to enter, leaning out the air/fuel mixture and leading to misfires.
  • Low Compression: Less common, but a serious issue like a worn piston ring or valve problem can cause low compression in a cylinder, leading to a misfire.
  • Carbon Buildup: Direct injection engines like yours (B48 engine in the F56 Cooper S) are prone to carbon buildup on the intake valves, which can restrict airflow and cause misfires, especially at idle.

Troubleshooting Steps You Can Take:

  1. Inspect Spark Plugs: If you're comfortable, remove the spark plug from cylinder 2 and inspect it. Look for signs of fouling, wear, or damage. You could also swap it with a plug from another cylinder (e.g., cylinder 1) and see if the misfire code moves to P0301.
  2. Test Ignition Coil: Similar to the spark plug, you can swap the ignition coil from cylinder 2 with one from another cylinder. If the P0302 code changes to, say, P0301, then you've identified a faulty coil.
  3. Check for Vacuum Leaks: Listen for hissing sounds around the engine bay, especially near the intake manifold. You can also use a smoke machine (best done by a professional) to detect leaks.

When to See a Shop:

If the simple spark plug or coil swap doesn't resolve the issue, or if you're not comfortable performing these checks yourself, it's best to take your Mini to a qualified mechanic. They have specialized diagnostic tools to:

  • Perform a compression test to rule out internal engine issues.
  • Test fuel injector operation and spray pattern.
  • Check for carbon buildup on intake valves (which might require walnut blasting).
  • Conduct a more thorough diagnostic scan to look for other related codes or sensor readings.

Driving with a persistent misfire can damage your catalytic converter over time due to unburnt fuel entering the exhaust, so it's important to address this issue promptly.

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