Plugging the hose increases pressure in the crankcase, which is functionally the equivalent of pulling a vacuum on your shut off valve. Hence, it shuts down.
Failing to shut down could be as simple as a problem with the shut off valve diaphragm or a bad gasket somewhere. Plugging the vent will have a tendency to blow seals as crankcase pressure increases.

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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