1993 300D 2.5 turbo BLACK smoke problem
 

I have a 1993 300D 2.5 turbo that had intermittant wastegate opening/closing problems before it finally stopped closing and now the car blows ALOT of BLACK smoke and has no power...feels like it has about 65 hp, like the old 220D. Talked to Dave M and he is not convinced that it is a wastegate problem and suggested that I post the get the "collective" to ruminate on this problem so I can solve it...You guys are AWESOME!!!

Could it perhaps be a clogged cat? Only I reason I suggest that as a possibility is that your symptoms are exactly what I was experiencing recently with my 95 E300D. Disconnected the exhaust, and voila, power was back and smoke was gone. Replaced the cat and everything works perfectly.