This is a vacuum issue and was an easy fix on my 124. I imagine it would be even easier on the 123. Also, I would imagine that their is a manual shut off switch near the IP too (anyone?). This will save you some time and diesel.

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1996 (W210) E300D 86K - Traded in for a Lexus
1992 (W124) 300D 2.5 Turbo 202K - Sold
1983 (W123) 300D, 146K - Sold
1970 280S, 263K - Sold - Beginning of addiction