Isolate the roughness to its cylinder by cracking loose each metal line at each injector, much like pulling a plug wire off a gas car. With it running, crack the line loose, a good injector will run much rougher when this is done. A faulty injector will not affect the idle roughness. That way you can confirm an injector issue. Would be good to know compression is even on all cylinders and that the valve adjustment was already done as well.

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81 300CD (sold) 1972 280 SEL 4.5 (sold) 1966 250 S 4 spd (sold) 1974 450 SL (sold) 86 BMW 325ES (sold), 1973 280C (sold) 1988 300 SE.