The shutoff vacuum valve is at the back of the injection pump, up against the oil filter, down below the pump. You can't see it but you can feel around down there. There should be a small pipe made of blue and brown plastic connected to it. This is the vacuum that shuts off the fuel.

Trace this pipe back to where it joins other pipes, and follow them all looking for leaks, cracked hoses, or hoses fallen off. When mine wouldn't shut down it turned out to be a connecting hose 2 inches long which had a crack in it, located at the far left front of the under hood area.

I also had a blockage in the main pipe where the small pipe connects up.

If there are no loose pipes or cracked hoses or blockages try the mityvac on the shutoff valve. If it won't kill the engine you need to replace the valve.