I think that your engine has gone into what is called the lump mode, if this is the problem the engine will not turn more than 2800 RMP's, and the engine is running at half horsepower. I had this problem with my 2006 Sprinter last week while in Co. This engine has a Turbo Resonator on the intake side of the turbo, this part has been know to crack, and this alows the turbo boost to drop, therefore your enging will go into lump mode to prevent the engine from being overfueled, if this is the problem the part is easy to replace, and only cost about $75.