XSeries-forum

I have seen many posts about folk experiencing it and it seems in most instances to be fairly normal. But my question is - what actually causes it. I have heard some say its 'sooty deposits' but if they are deposits why are they emitted via the exhaust just because of heavy acceleration. i heard some say unburnt fuel but would that come out as smoke?

So whats the verdict on what it is?

Smee

The internal combustion engine is not 100% efficient so your throttle foot demands 'go', the ECU says 'dump a load of fuel that the engine can't burn immediately' and the engine only burns what it can with the rest coming out as smoke because the mixture is too 'rich' until the rpm/fuel feed/airflow in all meet at the optimum point.

I think if you had one of those good old Sparkrite Colourtune devices you'd see the flame go from blue to yellow as you buried the throttle.  Light a fire with petrol and it's yellow flame with loads of black smoke.  Boost the air-flow and the flames got blue and you get less smoke - at least that's my theory.

Richard

You have got to dump the fuel in for the engine to rev faster, but as the X5 is a heavy, unaerodynamic 2 tonne lump. The engine cannot react fast enough to burn all the fuel efficiently. Some of the unburnt fuel gets burnt off in the cat and exhaust. Hence the black smoke.