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2018 Mini Cooper S Rough Idle and Misfire - CEL On

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Chris P. · rep 0 · May 22, 6:52 AM · 1 replies · solved · Question

Vehicle: 2018 Mini Cooper S Hardtop 2-Door

Hey everyone,

I'm having some trouble with my 2018 Mini Cooper S Hardtop 2-Door, and I'm hoping someone here might have some insight. Lately, it's developed a really rough idle, especially when cold, and I can feel it misfiring under light acceleration. The Check Engine Light (CEL) came on a couple of days ago, and it's making me pretty nervous.

I haven't had any major issues with it before, and I keep up with regular maintenance. The car has about 65,000 miles on it. It feels like it's losing power, and the exhaust note sounds a bit off. I'm worried it could be something serious.

Has anyone experienced similar symptoms with their Mini Cooper S of this generation? What should I be looking for? Any common culprits for rough idle and misfires on these engines? I'm trying to figure out if this is something I can tackle myself or if it's definitely a shop visit.

✓ Accepted solution

ClearTheCode Admin

ClearTheCode Team · rep 0 · May 22, 6:52 AM

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ClearTheCode admin answer:

A rough idle and misfire with a Check Engine Light (CEL) on your 2018 Mini Cooper S are common symptoms that can point to several engine-related issues. Given the mileage, it's wise to address this promptly to prevent further damage.

Here are the common causes and diagnostic steps:

  1. Spark Plugs and Ignition Coils: Worn spark plugs or failing ignition coils are very frequent causes of misfires. Over time, spark plugs degrade, and coils can fail, leading to an incomplete combustion cycle. This is often the first thing to check.
  2. Fuel Injectors: Clogged or faulty fuel injectors can lead to an improper fuel-air mixture, causing misfires. Fuel injector issues can sometimes be intermittent.
  3. Carbon Buildup: Direct injection engines, like those in your Mini Cooper S, are prone to carbon buildup on the intake valves. This buildup restricts airflow and can severely impact engine performance, leading to rough idle and misfires. Walnut blasting is a common solution for this.
  4. Vacuum Leaks: Leaks in the vacuum system can introduce unmetered air into the engine, disrupting the fuel-air ratio and causing a rough idle. Inspect all vacuum lines and connections.
  5. PCV Valve/System: A failing PCV (Positive Crankcase Ventilation) valve or a leak in the PCV system can also cause vacuum leaks and contribute to misfires and rough idling.

Diagnostics:

  • OBD-II Codes: The CEL indicates that your car's computer has stored diagnostic trouble codes (DTCs). Use an OBD-II scanner to retrieve these codes. Common misfire codes include:
    • P0300: Random/Multiple Cylinder Misfire Detected
    • P0301, P0302, P0303, P0304: Cylinder 1, 2, 3, or 4 Misfire Detected (indicating a specific cylinder)
  • These codes will help pinpoint the affected cylinder(s), narrowing down your diagnostic efforts.

When to See a Shop:

While checking spark plugs and coils can be a DIY task for some, diagnosing fuel injector issues, significant vacuum leaks, or especially carbon buildup often requires specialized tools and expertise. If the OBD-II codes point to multiple cylinders or if replacing plugs/coils doesn't resolve the issue, it's highly recommended to take your Mini to a qualified mechanic. Continuing to drive with a persistent misfire can lead to catalytic converter damage, which is a very expensive repair.

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