FYI, that picture's actually of a 300D...the "T" in "300TD" doesn't stand for "Turbo" like one would think
it actually stands for "Touring" or the German equivalent thereof, and was M-B's designation for a station wagon. A 300D is a sedan; a 300TD is a wagon, and a 300CD is a coupe.
They are most certainly NOT speed demons...I don't recall the exact "specification" 0-60 time, but I want to say that it's in the realm of 15 to 20 seconds. With that said, there are a few things worth looking at...first, with the engine off, have someone put the accelerator pedal to the floor and take a look at where the linkage connects to the injector pump, and see if you're getting full throttle. After about a year with my first 300D, I found that with the pedal all the way down, I had about 1/8" before the linkage was actually maxed out...I adjusted the linkage, and the difference was night-and-day.
A couple of other things...first, don't be afraid to do a WOT acceleration. These engines were designed to be driven by taxicab drivers with binary right feet
and the engine actually isn't happy unless it's given regular full-throttle accelerations. Also, the injectors could be dirty...they can be replaced, but I would start out by running a full-scale
Lubro Moly Diesel-Purge treatment through the fuel system, and see what that does. Doing a valve adjustment would also be a good idea. Lastly, run a few Italian tune-ups to make sure that the mufflers (yes, there are two
) are clear...when these cars are being driven like they're owned by my grandmother, the soot tends to clog the mufflers, and the best way to clear them out is to do a few full-throttle high-speed runs
Hope this helps some
...BTW, IIRC the GM 6.2l and 6.5l has a similar injector return line setup to the M-B's, and I think that it's actually a better design than what our IDI's use...less of a chance of air intrusion...