There is a vacuum operated shut off valve on the back of hte IP. pull off the brown vacuum line and it's out of the equation although they rarely fail in the 'off' position.
I agree. Your symptoms are typical of death in a jug. Some people have resuscitated such murder victims, one by running a solvent thru the IP. The other option is to swap in a different IP if you happen to have one laying around.

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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