You've got a manual shut off lever at the top of the linkage right near the valve cover. It should have a red label saying 'Shut OFF". There's a vacuum hose that goes to the shut off diaphram below that on the back of the IP. Trace it out. If it's disconnected, there's your problem. However, problems somewhere else in the vacuum system can also stop the shut off from working. To test the shut off itself, remove the original vacuum line, put a long piece of vacuum line in its place, start the engine and suck with your mouth on the vacuum line. A couple of good sucks should shut it down if your shut off diaphragm is good. If it shuts off ok that way, the vacuum problem is elsewhere.

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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