Mini One Diesel Performance Issues

Sounds like a faulty manifold flap to me. I had similar symptoms on my Mini One – reduced engine power, rough running, and even some jerking during acceleration. Did you check the airflow control system for any error codes? Is the hissing sound constant or only under load?

Interesting. How did you pinpoint the manifold flap as the culprit? Did you experience the higher fuel consumption as well, and was it hard to diagnose? Any specific tests or observations that led you to that conclusion?

The hissing sound is a giveaway, almost like a vacuum leak. My car had the exact same symptoms. I checked the intake system, and the manifold flap wasn't moving freely. A faulty switch intake manifold control was the issue. I'd advise getting it checked at a workshop; they'll be able to confirm the diagnosis.

Huge thanks! I took it to a workshop, and you were spot on. The intake manifold flap control was indeed defective. 465€ later, the Mini is running smoothly again. The engine performance is back, and the hissing sound is gone. Appreciate the help!