I did a quick check on my 300D last night.
I used my Mityvac to check out my vacuum shutoff valve. I can pull a vacuum and the engine shuts down. It will hold the vacuum and not allow the STOP lever to return to the UP position until the vacum is released.

I then reconnected the vacuum line to the shutoff valve. Started the car and then turned it off. The STOP lever dropped and the engine shut down. then the lever rose up again.

On the W123 at least, the vacuum valve on the ignition switch had 2 ports, and one has a vent hole associated with it. If the vent hole becomes blocked, it is possible that the vacuum valve will be applying a vacuum to the shutoff valve, but not alowing it to leak off and allow the STOP lever to return to the Run position.

Using a Mityvac, on the vacuum line to the shutoff valve, turn the engine off (it won't shutdown because you have the vacuum line off) then manually shutdown the engine using the STOP lever. The vacuum should be at maximum (less the leak) and then leak down. If not, you may have a blocked vent in the ignition shutdown valve.

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Tony from West Oz.
Fatmobile 3 84 300D 295kkm Silver grey/Blue int. 2 tank WVO - Recipient of TurboDesel engine.
Josephine '82 300D 390kkm White/Palamino int.
Elizabeth '81 280E, sporting a '79 300D engine.
Lucille '87 W124 300D non-turbo 6 cylinder OM603, Pearl Grey with light grey interior


Various parts cars including 280E, 230C & 300D in various states of disassembly.