Someone correct me if I'm wrong... There def is vacuum that runs from the ignition switch, which will feed that vacuum through to the shut off valve when the ignition is cut off. The vac line in the engine compartment is brown (at least on mine it is). If there is a bad connection there... or no connection at all then the car will remain running until you shut it off manually. If this line is connected and it all looks ok in the engine compartment, one way to try to shut your car down is to try locking the doors after you turn the ignition off. This will pretty much always shut mine down if she is slow to do so with just the ignition. And if that works *fingers crossed* then you have limted the vac leak to somewhere inside the car! Maybe under the dash, maybe under the carpet, maybe in the trunk! Time and a mityvac will tell. Good luck. Hope this helps at all.

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1981 300CD 260k w 60k on new motor
1983 240D 4spd Manual