Could be the engine or the turbo.
Is the engine burning oil
Pull out the dipstick with the engine running and if it's an engine problem you will be getting fumes coming out of it,you can also do the same at the oil filler cap.
If you have no fumes you're engine is ok and you probably have a turbo problem.
(Burning oil means low oil level in comparison to any evident leaks).
If that's ok Check turbo hoses'"the slightest split or crack in a hose can cause it to blow smoke.
Computer scan might help.
Before changing turbo get it tested by a transit specialist if you can find one.
Good luck
Mark
Mark, October 2011