Bad fuel injector is the cheap cause, bad fuel distributor is the expensive one.

I would replace at least that one injector and all the injector seals for a start. This may fix the problem completely, as a leaking injector can cause #1 to foul at idle. If the plug stays black, chances are you have a fuel distributor problem.

Lean running due to injector seal leaks on the other cylinders, or clogged internal filters resulting in low fuel delivery will also make #1 run over rich, so a complete set of injectors isn't a bad idea either. Cannot usually clean them once plugged.

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!