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.