Well, it is a system.

Disconnect the vacuum line at the fuel injection pump shut off. If there is oil inside the tube the diaphragm is porous. However, like on mine it is porous but still works. You don't need a mighty vac to see if it will shut off. Connect a clean length of vacuum line and simply suck on it. It is supposed to shutoff as was said.

The mighty vac (handheld vacuum tester) is good to see if the pump shutoff will hold vacuum.

Next there is a vacuum shutoff switch on top of your steering Colum lock. That can leak or malfunction. Have not read of a way to test that.

Other vacuum leaks else wear can keep your engine from shutting off.

Concerning the manual shut off all of the linkages are supposed to have specific lengths.

If you have an MW Type fuel injection pump when you use the shutoff it pushes up against a spring-loaded rod. If the part of the rod that is inside of the tube is rusted the rod may not move as far as it is supposed to go to shut off quickly.

Obviously, it any of the tubing is cracked it can leak vacuum. Those 2 and 4 way rubber connectors tend to do that.

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84 300D, 82 Volvo 244Gl Diesel