I have been looking for a while at several W123 diesels, and test driven quite a few. Today I looked at a car that seems in quite good condition in most respects, checked all the "normal" issue spots. Drive-train seems in general good condition with very little engine blow-by. No smoke on start-up and smooth idle. Great body and interior.

The one thing that stands out seems to be a lack of power coming off idle (this is in comparison to the other OM617 turbos that I have driven). There seems to be almost no torque until about 1500 rpm, at which time the engine responds normally. It does not matter if accelerator is all the way down, no black smoke or power.... At these rpm's, the turbo should not have any effect, so I think I can rule that out.

Any thoughts on what could be going on? Seems that there is insufficient fuel delivery for some reason.

Thanks for any comments

 

I say too much play in the Throttle Linkages.
Check the Bell Crank On the Fire Wall and see if when you step on the Pedal there is a lot of movement of the linkage before the Throttle Arm on the Fuel Injection Pump moves.
Step all the way down on the pedal and see if the Throttle Arm bottoms out on the Throttle Stop Screw.

 

Throttle linkage is a good place to start. Check the fuel filters and screen for blockage. Also, the banjo bolt on the backside of the intake manifold is probably clogged up. Brake cleaner will blast that gunk out. Also, the line from that banjo bolt to the over boost protection valve can be clogged. Again, blast brake cleaner and run a length of wire through it. Check for oil puddling in the turbo. That's a sign of bad seals. The turbo doesn't fully spool until around 2k rpm but will start building boost around 1200 rpm. Any smoke?

 

As a Fuel Filter gets gradually restricted it generally would start slowing you down at the top end first and the OP is having the revers of that.

There is a issues with the Fuel Supply Pump and the Fuel Pressure Relief/Overflow Valve that can also slow the Car a bit but that often effects the idle and the OP said the it idled smoothly.