Mini Cabrio Air Intake Leak Suspected

Sounds like a vacuum leak, I had a similar issue with my old Opel Astra. High fuel consumption and engine misfire were the main symptoms. Check all the vacuum hoses connected to the intake manifold and throttle body for cracks or loose connections. A loud hissing noise when you accelerate definitely points to a leak in the air intake system.

Thank you. Where exactly should I look for these vacuum hoses? Are they easy to identify, and is there a way to test them without special equipment?

They are usually small, rubber or plastic hoses connected to the intake manifold. Some go to the brake booster, others to various sensors or the PCV valve. Given the symptoms, a visual inspection might reveal cracks or splits. You can try spraying some carb cleaner or soapy water around the hoses while the engine is running. If the engine rpm changes or you see bubbles, you've found a leak. Given the age of your car, it's likely a vacuum hose has perished. But I suggest you go to the workshop to perform a leak detection test.

Just an update: I went to the workshop. It was indeed a cracked intake hose. They replaced it, cleared the check engine light, and the hissing sound is gone. Fuel consumption seems to be back to normal. Thanks for your help!