This is per my 1981 300SD but might relate to your 300D - not really sure but maybe.

Sounds like a bad shut off valve. It is located inside the injection pump at the back near the firewall. You need to remove the VCV - vacuum control valve - (carefully) just above to get to it. The shutoff valve has a hook; when you turn the ignition off, vac is supposed to actuate the hook and shut down the engine. However you might test it first - hook up a Mityvac to the nipple on the shut off valve and start the engine - if you pump up the Mityvac it should immediately stop the engine. If not, the shut off valve leaks and needs to be replaced.

It could also be the two nipple ignition vac switch but the almost never fails.


Try this: How to Repair Your Shutoff Valve https://dieselgiant.com/repairyourshutoffvalve.htm