Mini One Hissing Sound on Acceleration

That sounds awfully familiar! I had a similar issue with my old Volkswagen, same symptoms: check engine light, weird hissing sound on acceleration, and the fuel economy went down the drain. For me, it turned out to be a split vacuum hose connected to the air intake manifold. Have you checked all the vacuum lines thoroughly? Sometimes it's a small crack that's hard to spot. Does the engine misfire at all?

Okay, so the vacuum hoses are typically thin, black rubber tubes running from the intake manifold to various components like the brake booster, fuel pressure regulator, and other emission control devices. Trace them carefully, feeling for cracks or splits. A misfire feels like a slight jerk or hesitation during acceleration. It could also cause a rough idle. Given your symptoms and the age of the car, I'd say a perished rubber hose is highly likely. If you're not comfortable doing it yourself, I'd suggest taking it to a workshop. They can do a smoke test to pinpoint the leak.

Thanks, that's helpful. Where exactly should I be looking for these vacuum hoses? I've had a quick look around the air intake and air filter, but I can't see anything obvious. Also, what does an engine misfire feel like? I'm not sure I've experienced that before.

You were spot on! Took it to a local mechanic, and it was indeed a cracked intake hose. Replaced it, and the hissing is gone, the check engine light is off, and my fuel consumption is back to normal. Total cost was just under 100€. Thanks a bunch for your help!