It can be as simple as a small hole or slack jubilee clip in the air pipe between the flow sensor and the air intake. It happened to my sister's Volvo and also a mate's Zafira with a few weeks of each other a few years back.
The escaping air ( which makes a right racket as it's pushed through at high pressure by the turbo) means that not all the air going through the flow sensor goes into the engine. There is then far too much fuel for the amount of available air, meaning lots of black smoke!
In both cases I'm aware off, after replacing a £1.50 part, the car ran fine for another 50k miles at least.
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