Test the shut off valve itself with a mighty vac, or just by connecting a length of vacuum hose and sucking. Testing the line tells you whether or not the valve behind the ignition switch is good. A good shut off valve needs less than 5" of vacuum to shut down the engine. I'd leave it in there as long as it works. When the diaphragm is really gone the vac line will start to suck oil out of the injection pump.

If you turn the ignition key back to the run position after the engine shuts down, it will zero the vacuum out on that circuit-- assuming that all of the check valves are functioning properly. That may help to extend the life of a weak diaphragm for a while.

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'79 240D