Black smoke off idle at low load usually means low compression, unless you are driving a VW/Audi, in which case there is always a small "flag" of black smoke.

Black smoke under load has too many causes to come up with a simple diagnosis without more info!

Peter

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!