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2020 JCW Reduced Power & Turbo Underboost (P0299)

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Chris P. · rep 0 · May 25, 4:55 AM · 1 replies · solved · Question

Vehicle: 2020 Mini Cooper JCW Convertible

Hi Mini forum,

I'm having an issue with my 2020 Mini Cooper JCW Convertible. The Check Engine Light came on, and I've noticed a significant reduction in power, especially when trying to accelerate hard. It feels like the turbo isn't kicking in at all.

I scanned it and got a P0299 code, which points to 'Turbocharger/Supercharger Underboost'. This is pretty concerning for a JCW! I've read a bit online, and it could be a few things, but I'm not sure where to start looking.

Has anyone else experienced this with their newer JCW? What were the common culprits?

✓ Accepted solution

ClearTheCode Admin

ClearTheCode Team · rep 0 · May 25, 4:55 AM

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ClearTheCode admin answer:

The P0299 'Turbocharger/Supercharger Underboost' code on your 2020 Mini Cooper JCW Convertible is a common diagnostic trouble code that indicates the turbocharger is not producing the expected boost pressure. This directly leads to the reduced power you're experiencing.

Here are the most common causes and diagnostic steps:

  1. Vacuum Leaks in Boost Control System:

    • Many turbo systems rely on vacuum lines to control the wastegate actuator. Inspect all vacuum lines connected to the turbo, wastegate, and boost solenoid for cracks, disconnections, or deterioration. Even a small leak can prevent proper wastegate operation.

  2. Wastegate Actuator/Valve Issues:

    • The wastegate controls exhaust flow to the turbo. If the actuator or the wastegate valve itself is stuck open, exhaust gases bypass the turbine, leading to underboost. Visually inspect the wastegate linkage for freedom of movement and proper connection to the actuator.

  3. Boost Pressure Sensor (MAP Sensor):

    • A faulty Manifold Absolute Pressure (MAP) sensor can send incorrect boost readings to the ECU, causing it to detect an underboost condition even if the turbo is functioning correctly. Cleaning or replacing this sensor can sometimes resolve the issue.

  4. Charge Pipe Leaks:

    • The charge pipes carry pressurized air from the turbo to the intercooler and then to the intake manifold. Look for cracks, loose clamps, or disconnected pipes in this system. A boost leak test (pressurizing the intake system) is the most effective way to find these.

  5. Turbocharger Failure:

    • In more severe cases, the turbocharger itself could be failing. This might manifest as excessive shaft play, oil leaks, or unusual noises. However, always rule out simpler issues first.

When to See a Shop:

Given that this is a performance-oriented JCW model, and turbo systems can be complex, it's highly recommended to have a Mini specialist diagnose this if you can't find an obvious vacuum or charge pipe leak. They have specialized tools to monitor live boost pressure, perform actuator tests, and pinpoint the exact component causing the underboost, preventing potential further damage to the turbo or engine.

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