I believe the 93 300D 2.5 has the glow system that stays powered for some time after the engine is running. This should eliminiate incomplete combustion after a cold start. Does the glow light come on for a few seconds then extinguish? Do you wait for the glow light to extinguish before starting the engine? Sorry to ask, but we should not assume. I don't know how to test the "after glow" feature except by checking that the glow plugs are drawing 50 amps or so.

I suspect there are one or two lazy glow plugs in the mix. You can check resistance between ground and the individual glow plug pins in the bigger glow plug relay connector. The acceptable resistance value for glow plugs is in the archives.

Perhaps a 93 E300D is a different car than I'm thinking it is.

Sixto