Hey guys! I'm new here and don't know a whole lot about car repair. I read the articles on limp mode and general power loss on TDI's and the common causes, but I'm a little over my head. I've already brought it to a mechanic, and he tried replacing a turbo valve (?) and the fuel filter, which seemed to help some but not completely. He said the fuel filter was really bad, so he recommended I put some additive into my fuel to clean it out which I have done. Whereas I'm used to feeling a surge of power when I get to about 1700 RPM, presumable when the turbo hits, now I'm only getting real power starting at about 2500 RPM and even then it's a bit weaker. I also don't hear any turbo whistle like I normally do. The engine light is not on, and IIRC the mechanic said it hadn't thrown any codes. I have a good bit of black smoke coming out the back when driving, especially up hills. Any help would be extremely appreciated.