With it at idle crack loose each hard line one at a time to hopefully isolate the cylinder thats different. Crack the line loose at the injector. Its much like shorting out a plug wire on a gas vehicle. Once cracked, there should be a noticable rougher running engine. Move down the line till the one you loosen doesn't affect the idle as much. Thats the cylinder thats acting up. Investigate for partially plugged injector, tight valves etc. If all seem the same then look for something attached to the engine thats also touching the body or frame, exhaust collapsed mount, air cleaner housing, etc.

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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.