I have an 82 240D - it doesn't have the rack damper bolt that is on the 300D's. Not sure about the pre-123's.

Another cause of rough idle that you can check is cylinder compression. Compression should be > 200PSI (Preferrably 400PSI) and fairly even across the four cylinders. My guess is one may be a bit low (especially if it starts okay).

Good luck.

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1976 240D
1987 560SL
2007 E320 Bluetec
1998 C280 (now son’s car)
1982 240D Manual - Sold