Injection pump timing, unbalanced injectors, worn delivery valve seals, lots of other things. Check the Diesel Discussion section archives.

Run a couple of cans of Diesel Purge if you haven't done it in a while.

The MB CD manual is about the only thing you'll find. There's some engine coverage in the Haynes W124 manual but nothing to help you diagnose smoke. I don't know if there is engine coverage in the Ritter or Technibooks volumes on the W124.

Sixto
91 300SE
87 300SDL
83 300SD