check the shut off valve on the back of the injection pump.

you'll need a mityvac or similar vacuum pump/tester. pull some vacuum on the pump, if the car shuts off then the valve is good and you'll need to trace the vacuum lines back until you find the leak. i dont remember exactly how much vacuum the valve needs to activate but I believe it's not a lot, i think 10 or 15" or something.

it's possible its the vacuum switch at the ignition. someone else would have to chime in on how to check that. my guess would be, turn the key to "off" and then apply vacuum to the supply line to the ignition vacuum switch. if it doesnt help or "slows" things down, it could be a break in the line or a bad ignition shutoff switch.

the vacuum circuit is should be easy to troubleshoot, because at least on my 81 300D, it's basically its own circuit. it should be supplied directly from the brake booster hose, and the line are brown. a brown line should go from the booster supply into the car through the firewall. another brown line should come out from the firewall and go to the shutoff valve on the back of the injection pump.