Probably motor mounts. With it running crack loose one injector line at a time at the top of each injector. If the idle is affected evenly as each line is cracked then re tightened , then the injectors are probably ok. Loosening each line is like pulling each spark plug wire off a gasser. If the drop in RPM is the same line to line, then its firing on all 5.

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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.