My car (1998) E300 TDT is having this harch / hard sound. As far as I remember the 6er should run much more smooth. So I wonder if it can be a worn out injector.

It runs a little cold now, so I will exchange the thermostat. Some says that it running cold may cause the harch sound.

But I think it might be either an injector, or that is sucks air in the diesel returns (my workshop said that might be a problem).

Is there a way that I can check the injectors myself?

For the record: I also have problems with air in the fuel lines, and am waiting for spare parts. Can air in the fuel lines cause sounds too, or is the air somehow removed from the fuel before it goed to the injectors?