On my 240 D ...If the shutofff valve is attached wrong...it holds the rack full forward resulting in full throttle only....

The arm of the shutoff valve on mine has a little (L) shape on the end that fits into a slot in the rack slider mechanism.....inside the IP Gov assy.

If the shutoff valve is just pushed onto the IP Gov. Assy without hooking it in the slot... the (L) hits the end of the slider mechanism pushing rack forward ...(full throttle)

(L) can not pull rack to off position because it is not attached.....

Again this is on a 1980 240D ....yours may be similar....

It is possible to reach the vac shutoff key assy under the dash on mine.....remove knee bolster for access...

Also another test to see if your vac pod on the IP can shut off the IP is to suck on the line attached to the shutoff pod.....only takes about 5 inches of vac to pull the rack shut....
If this does not shutoff engine ...check attachment inside IP......

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1978 Yellow 300D (The Mustard Toad)
1980 Blue 240D (The Iron Toad)
1989 Grey Mitsu.4WD Mighty Max Pickup (Needs a Diesel transplant bad)

(Open the pod bay doors HAL)