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2018 Mini Cooper S Rough Idle and Misfire - P030X Codes

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Jason D. · rep 0 · May 22, 6:54 AM · 1 replies · solved · Question

Vehicle: 2018 Mini Cooper S

Hey everyone, I'm having some trouble with my 2018 Mini Cooper S. Lately, it's developed a really rough idle, especially when cold, and the engine feels like it's misfiring under acceleration. The Check Engine Light came on, and when I scanned it, I got multiple P030X codes – specifically P0301 and P0302, indicating misfires on cylinders 1 and 2.

I've got about 60,000 miles on it. I recently had the spark plugs replaced about 10k miles ago, but I'm wondering if it could be coil packs or something else. The car also feels a bit sluggish, not its usual zippy self.

I've checked the oil level, and it's fine. No strange noises from the engine bay, just the rough running. Any ideas on what I should check next before taking it to the dealer? Thanks!

✓ Accepted solution

ClearTheCode Admin

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

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ClearTheCode admin answer: Misfires and rough idle, especially with P030X codes, are common issues that can stem from several sources in your 2018 Mini Cooper S. Given the direct injection engine, there are a few specific areas to investigate.

Here's a breakdown of potential causes and diagnostic steps:

  1. Ignition System Components:

    • Spark Plugs: Even if recently replaced, a faulty plug or incorrect gap can cause misfires. Inspect them for fouling, damage, or incorrect type.
    • Ignition Coils: These are a very common failure point on Minis. A failing coil pack will often cause a misfire on a specific cylinder. You can try swapping the coil packs between the misfiring cylinders (P0301, P0302) and a known good cylinder. If the misfire code follows the coil pack, you've found your culprit.

  2. Fuel System:

    • Fuel Injectors: Clogged or failing fuel injectors can lead to an improper fuel spray pattern, causing misfires. This is more common with higher mileage. A fuel system cleaner might help, but a failing injector usually needs replacement.
    • Fuel Pressure: Low fuel pressure can affect all cylinders, but sometimes manifests more prominently in certain ones. This would require a fuel pressure test.

  3. Air/Vacuum Leaks:

    • Intake Manifold/PCV System: Vacuum leaks can introduce unmetered air, leaning out the air-fuel mixture and causing misfires, especially at idle. Check all vacuum lines and the PCV valve/hoses for cracks or disconnections.

  4. Carbon Buildup:

    • Intake Valves: Direct injection engines like in your Mini are prone to carbon buildup on the intake valves because fuel is injected directly into the cylinder, bypassing the valves. This buildup restricts airflow and can cause significant misfires, rough idle, and reduced power. If other diagnostics don't yield results, carbon cleaning (often via walnut blasting) is a strong possibility for your mileage.

When to see a shop:

  • If swapping ignition coils doesn't move the misfire, indicating the issue isn't with the coil itself.
  • If you suspect clogged fuel injectors or low fuel pressure, as these require specialized tools for testing and replacement.
  • If you suspect significant carbon buildup. Walnut blasting is a specialized procedure best performed by a qualified technician.
  • If the misfire is persistent and severe, as continued driving can damage the catalytic converter, leading to a much more expensive repair.
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