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2018 Cooper S Rough Idle and Stalling - CEL On!

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Chris P. · rep 0 · May 22, 6:54 AM · 1 replies · solved · Question

Vehicle: 2018 Mini Cooper S

Hey everyone, hoping someone here can help me out. I've got a 2018 Mini Cooper S, the F56 model, with about 65k miles on it. Lately, it's been acting really strange.

When I start it up, especially cold, the idle is super rough, almost like it's going to stall. Sometimes it actually does stall if I don't give it a little gas. The check engine light came on a few days ago and hasn't gone off.

It feels a bit down on power too, not as zippy as it used to be. I'm wondering if this is a common issue with these B48 engines? Could it be spark plugs, or something more serious? Any advice on what I should check first before taking it to the dealer?

✓ Accepted solution

ClearTheCode Admin

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

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ClearTheCode admin answer:

It sounds like your 2018 Mini Cooper S is experiencing symptoms consistent with an engine misfire or a fuel/air mixture issue, which are common concerns for the B48 engine in the F56 generation. The check engine light (CEL) confirms a fault has been detected by the engine control unit (ECU).

Here are the typical steps and potential causes:

  1. Retrieve OBD-II Codes: The first and most crucial step is to get the diagnostic trouble codes (DTCs) read from your vehicle's OBD-II port. Common codes for these symptoms include:

    • P030x (e.g., P0301, P0302, P0303, P0304): Misfire detected in cylinder X.
    • P0171: System too lean (Bank 1).
    • P0420: Catalyst system efficiency below threshold (Bank 1) - less likely to be the primary cause of rough idle but can appear if misfires are prolonged.

  2. Common Causes for Rough Idle/Misfires:

    • Ignition System: Worn spark plugs or failing ignition coils are very common culprits, especially around 60k-70k miles.
    • Fuel System: Clogged fuel injectors can lead to an improper fuel spray pattern, causing misfires. A failing high-pressure fuel pump (HPFP) could also be a factor, though less common on newer models for simple misfires.
    • Vacuum Leaks: Leaks in the intake manifold, PCV (Positive Crankcase Ventilation) valve, or vacuum lines can introduce unmetered air, leading to a lean condition and rough idle.
    • Carbon Buildup: Direct injection engines like the B48 are prone to carbon buildup on the intake valves, which can restrict airflow and cause misfires, especially at idle.
    • Oxygen Sensors: A faulty upstream oxygen sensor can send incorrect data to the ECU, leading to an improper fuel mixture.

  3. When to See a Shop:

    • Immediately: If the check engine light is flashing, this indicates a severe misfire that can damage the catalytic converter. Stop driving and get it towed to a reputable Mini specialist or dealership.
    • Soon: If the CEL is solid and the car is running rough but drivable, you should schedule an appointment as soon as possible. Continuing to drive with a persistent misfire can lead to further engine damage or catalytic converter failure.
    • DIY Diagnostics: If you have an OBD-II scanner, you can pull the codes yourself. For basic checks like spark plugs, some DIYers are comfortable, but for deeper diagnostics like fuel pressure tests, smoke tests for vacuum leaks, or carbon cleaning, professional tools and expertise are required.

Given the symptoms and mileage, starting with checking the ignition system (plugs and coils) and then looking into potential carbon buildup or vacuum leaks would be logical steps for a professional technician.

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