My engine (OM617 turbo) is throwing out a fair amount of black smoke at full throttle on the highway. Under normal conditions though, smoke is not obvious either visibly or by smell.

What does this indicate and what can I do, if anything, to minimize it?

The car is a 1983 California 300D Turbo that drives wonderfully. Good fuel economy, decent power, no oil consumption to speak of.

Thanks for any insight out there.