There are not that many things that a Diesel needs to run but fuel, air and compression are essential. So start by checking these three things first.
The fuel system has a screen at the tank, a prefilter just before the fuel enters the low pressure fuel pump, and a big one that sits just inboard of the power steering pump that filters fuel before it enters the injection pump. Any one of them can limit flow, but the last one is where fine particlulate and water collect. This one resembles a spin-on oil filter. If you checked the tank element recently it is probably still ok, so examine the inline unit, ususally clear plastic but they come in translucent white sometimes, and if you have not replaced the spin on unit, do so. Also check for any signs of Diesel fuel leakage. This is usually accompanied by the aroma of Diesel fuel and visual wetness on lines and in the general area.
Check the throttle linkages for free movement and how far they move. I had the section that translates the pedal motion, linear, to rotational motion to twist the connector to the pump linkages go bad. So, about 70% of the pedal stroke was taken up by the slop in the system. Apparently an old fuel leak damaged the rubber bonded to the mechanical linkage and the rubber fell out. I temporarily fixed it with popsicle sticks and electrical tape, and it holds about six months to a year. I have yet to identify the parts that have failed on the dealer's microfiche so I have not ordered the part. But not having a good stroke on the throttle linkage at the injection pump is going to limit the fuel flow as well as a clogged or blocked line.
The air system is much more straight forward. Clean filter, check for any signs of rodent nesting or the like in the inlet, and make sure the throttle linkage that goes to the passenger side of the car and connects to the intake manifold, is working freely. This linkage controls a butterfly like valve plate in the inlet manifold, which is supposed to be fully open when you come off idle.
Compression is a little more of a challenge to correct if there is something wrong, but it is also the least likely to go wrong on a reasonably well maintained car. Compression can be adversely affected by valve clearances being out of specification, as well as a number of other, less adjustable, features, like fits between rings, pistons and cylinders, head gaskets and valve seating/sealing. Since you did not bring up oil usage, hard starting, or smoking, I would assume you have good compression and you should focus on the other areas.
This should help you get started, and good luck. Jim
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles
1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)