I have experienced on several occasions where the engine would not shut-off due to a vacuum leak and I have been able to to repair the problem. My experience is that it was always a door lock actuator and once the vacuum problem started it remained until it was fixed. This time my 1981 300D is the car with the no shut-off problem, however, it is intermittent and rare. When it does happen it will shut-off if I lock the doors. This cues me that it is a door lock actuator, however, I am baffled as to why it would not be constant. Could there be another culprit? Has anyone else experienced intermittent no shut-off on W123?

Don't be baffled. When rubber components (such as door lock actuator diaphragms) are involved, vacuum leaks are often intermittent in nature.

If locking the doors eliminates the problem, the chances are pretty good that you have a leak somewhere in the "unlock" circuit.

Could be anywhere in the unlock circuit. When I had a similar problem, I found the problem was a crack in the hard plastic line where it exits the driver's door and goes into pillar. A bear to fix. Cut the line and put on a rubber connector.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13