There are a number of causes. That is why you really need to pay to get a diagnostics readout. Stuck open cold start valve, dead O2 sensor reading too much oxygen all the time (although the usual failure is to read nothing at all, way rich), bad fuel distributor (that one is expensive), bad leak in the fuel distributor plunger, bad EHA (the latter is fairly common).

A leaking injector will cause one cylinder to be rich, but I'd be surprised if all of them were bad at one.

Do check for good wires and plugs -- bad ignition due to carbon tracks in the cap or a bad rotor can give you black plugs too.

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!