The engine of my 1984 300d turbo diesel will not stop running even after I switch off the ignition key. I have to push the STOP ENGINE lever in the engine compartment to turn it off. What is causing this condition and how can I fix it?

 

Your Car uses Vacuum to shut the Engine off.
Most often the cause is a Vacuum Leak in the Door Lock or Climate Control System.

You also have a Vacuum Shutoff Valve on the Upper Rear of the Fuel Injection Pump; it has only 1 Vacuum Line going to it and is all Metal. Look for any disconnected or cracked Vacuum Hoses under the Hood/Bonnet.
You have another Vacuum Valve that is actuated by your Key. It is mounted on the Steering Colum Lock.

If you are also have a problem of haveing to brake hard your Vacuum Pump itself could be going bad.
I will leave you a visator Message with more info.

 

Your shut off valve behind the fuel pump is defective.Have a vacuum pump on hand and check the vacuum at the valve itself.If it does not hold vacuum it need to be changed.This is a well known problem with Diesel engines in older model Mercedes Benz.