Put a vacuum hose in place of the brown vacuum line going to your shut off valve on the back of the IP between the IP and oil filter housing. With the engine running give a couple of strong sucks on the hose. If the engine shuts down, this confirms your shut off valve is functioning correctly. Next, I would disconnect the yellow vacuum line from the system at the firewall above and to the left of the of the oil filter housing at the firewall. Then block off the hose it plugged into (at the front end) with a golf tee. This will remove your door lock system from the equation. If the car shuts off the withe yellow line removed and plugged, this proves a leak in the door lock system is lowering the vacuum level ehough to stop it from shutting off.
If it still won't shut off with the door locks disconnected, push the far right button on the climate control. This will eliminate a lot of the climate control vacuum pods. If it shuts off fine with the button pushed, it proves a leak in the climate control system is causing the problem. You could also try this before the door lock disconnection.
These are all test that don't require a Mity Vac.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13