Prior to my current 300D I had a VW Jetta diesel, that acted like what you described. Turned out the fuel pump inside the injector pump was worn out (this is what sucks fuel from the tank before it goes to the high pressure injector lines). I removed the injector pump (this was not an easy job) and had it rebuilt by a specialized diesel injector pump shop. Cost $500 which was 1/3 cost of buying a remanufactured pump. I put the injector pump back in, bled the injector lines, and the car started immediately! Wear on these pumps can be caused by Govt.-mandated low sulfur fuel that does not have good lubricity. Question is, how do you know if the injector pump is bad? If you are cranking the engine and the pump is working but the car is not starting, normally you will have fumes/vapors of unburnt diesel fuel coming out of the tailpipe. If nothing is coming out of the exhaust during cranking, look for a fuel problem.
Another thing that brought down yet another diesel I had (1980 Olds 88 with 350 cu-in diesel) was fungus growing in the tank that clogged up the fuel sock. The dealer's expensive solution was to drop the tank, drain and flush it. A cheaper solution is to buy a tube of diesel anti-fungus stuff available at many truck stops (on two other occasions I did this and it worked). It kills the fungus; then you should replace the fuel filter because the dead fungus can clog it up. Fungus can come from many sources, including condensed water in the bottom of the tank where fungus and bacteria grow, or could be pumped in with fuel if you buy diesel from a low-volume place (always buy diesel fuel from a high volume truck stop if possible!)
Hope this helps... Bill
- 1991 300D Turbo 2.5
- 2000 ML 320