1) Your car didn't come from the factory with electric fuel pumps. If it has them, it's part of a WVO conversion or someone got creative.

2) It doesn't hurt the engine to run without electricity. I don't know about the alternator. You should be more concerned with the safety and legal aspects of that practice. Removing the key locks the steering wheel but not the shifter!

3) Time for plan B. Seems you're not the type who'd have a MityVac. We can work around that...

Try this - locate the 5 way fitting atop the engine vacuum pump at the front of the engine. It should be inboard of the big fuel filter. Replace one line, any line, with a hose that will reach the injection pump and remove the hose that attaches to the shut-off actuator above the stop lever. Start the engine. Connect the hose from the vacuum pump to the shut-off actuator. If the engine stops, your problem is between the fitting along the brake booser vacuum line and the key switch. If the engine doesn't stop or is slow to stop, your problem is the shut-off actuator.

Sixto
87 300D