C220 CDI '99 model injection problem

[Kamakazi]

Hey guys, I hope you are all with clear minds to help me with my friends car. He is a mechanic himself only for Mercedes, but he can't solve this problem in his car. Okay, here are the symptoms of the problem:

1) Constant loss of power
2) Heavy black smoke
3) Consumes a lot of diesel
4) A shiver (shake) in the engine when it heats a bit
5) Very weak/slow start

He constantly has to change the four injectors. Once he changes them, the car goes back to normal for a maximum period of time of one week and then it brakes down again. Since he is a mechanic he uses used injectors since he knows that they will brake down.
He had cleaned the exhaust system and removed the hard filter (not sure this is what they call it), and there was no imporvement in the cars performance. He has also changed the turbo and still with no imporvement. He changed the turbo after several tries of changing the injectors, thinking that the turbo might be the problem, but again with no success in tackling this problem.

[therock2689]

Maybe something may be wrong with the wiring. Maybe some sensor is f'ed up and might be dumping in to much fuel. It may be running pig rich and causesing seveal systems to fail with it. If its running rich, is may be a MAS. Now, im not to familiar with the MB diesel, but i would think it has some kind of MAS. Teh shaking you said may be due to teh richness when teh glow plugs shut off and teh comprestion alone cant combust the fuel. Most if not all those symtoms make me lean towards a exsesivly rich a/f mix. If posible, hook up an a/f meter and see what it says, i dont know norm for a diesel, and tehn compare to a normal and see what you get.
Basicly after all that crap, you car is taking diesel shot and its getting all ten different kinds of f'ed up, if that makes any sence.