Good to have your experience on the results from replacing the injectors and glow plugs. Perhaps I'll leave mine alone then, as the glow plugs all test good now.
The car starts immediately and runs well. Then at just about the time when the glow plugs would be cooling down from "red hot" the idle gets a little rough. And the unburned diesel smoke starts. So, I run the engine speed up to about 1500 rpm and let it smooth out. It is interesting that some people say they don't have this idle/smoke issue. For me, I can live with it easily given all the other great things about the car.
I find it interesting that my '82 300D does none of this. It idles smoothly right upon startup, and little if any unburned diesel smoke.
How did you manage to manually turn on the glow plugs for 20 seconds after start? Did you trick the glow plug relay somehow or did you direct-wire all the glow plugs to the battery? Or something else?
The other thing that occurred to me: I wonder if the glow plug relay from a later 603 engine could be retrofitted into the 1987 300D? I am assuming that it is the glow plug relay itself that contains the "after glow" control logic. Frankly, I'm not sure its worth $150 or whatever the GP relay costs to make the change. But if the relay failed it might be better to use the later part.