You can isolate which cylinder is the culprit by loosening the hard injector lines at the injectors, one by one. IIRC, it takes an open end 14mm wrench. You loosen the line just enough to have fuel spurt out and make note of the change in idle - if it gets worse, that's typical, if it doesn't change, you have your troublesome cylinder.

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1982 240D, sold 9/17/2008
1987 300D TurboW124.133 - 603.960, 722.317 - Smoke Silver Metallic / Medium Red (702/177), acquired 8/15/2009
262,715 and counting