Which one of your cars is this happening to?

If you can easily pop off the lnkage right at the shutoff lever; pop it off and try push the shutoff lever by hand and see if it shuts off.

Dose the car in question have a Mechanical Governor or is it an Pneumatic Governor (you will have a Butterfly Valve in your Intake Manifold and a linkage going from the Intake Manifold down to the iP).

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84 300D, 82 Volvo 244Gl Diesel