Petrol 5 series + black smoke

[Hyperion]

My sister has just bought a 2.0 litre petrol 5 series touring automatic.
It's done 150'000 miles on a 2000 V plate. She paid £4.5. It has a full service history.
No, I woulden't either, but she's a girl...

Anyway:
Going down a hill, foot hovering off the accelerator.
When she puts here foot back on the gas, the car bellows black smoke
The exhaust tip is very sooty and it appears to be using oil.

I guess the cars flocked...but what specifically is wrong with it?
Is it common?
Will it get to the point of being undrivable?
Or should she sell sell sell...

[dxb335d]

Black smoke is over-fueling mate. Blue smoke wouldbe oil burning.

I would get the emmissions checked firstly. Is it actually burning oil also?

Does it smoke on startup also?

[Hyperion]

dxb335d said:

Black smoke is over-fueling mate. Blue smoke wouldbe oil burning.

I would get the emmissions checked firstly. Is it actually burning oil also?

Does it smoke on startup also?

No, on startup it's clear. It only seems to happen when you accererate after going down a hill.

[Vixpy1]

Probably the Air flow meter.