Mercedes: Merecedes 1979 240D, fuel air mixture, spontaneous combustion

Question
Engine continues to run after switching engine off.

Answer
Sharon,
How do you get the engine to stop? Does it have to run completely out of diesel fuel?
Because the diesel has no  spark plugs and uses spontaneous combustion by squeezing the fuel air mixture, the only way to stop the engine is to interrupt the air or fuel.
There is an air valve that must be stuck or have lost its electrical connection to the starter switch.
I would locate a manual for the car and determine the location of the stop air solenoid.
Good luck and let me know if you have success.
Rob