Look for a big vac leak -- I believe that the engine runs rich with the frequency valve disconnected or bad (way rich, that is, with clouds of black smoke). If it runs OK with the valve unplugged, chances are the mixture is actually very lean -- fix that (vac leak) and it may be OK.

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!