The Vacuum Shutoff on the Fuel Injection Pump pulls the Fuel Rack into the shut off position. It won't work well if there is a Vacuum Leak in the system or the vacuum pump is not supplying enough vacuum.

Disconnect the hose at the Vacuum shutoff and see if there is Oil residue in it. If there is the diaphragm in the shutoof has holes and is passing oil through it. However, you can see oil and sometimes they still work normally.
As viewed from the drivers seat on the top right of the Steering Colum Lock is a vacuum shutoff valve. That could be worn out. Also the hoses on it need to go in the proper position. If you swap the positions it willnot work well.

You can have a vacuum leak anywhere in the vacuum system and it can effect the shutoff. To check the vacuum sustem you pretty much block disconnect one part of the vacuum system and plug the vacuum source and see if your vacuum situation improves. If it does the section you disconnected is the source of the leak.

For the under the hood you can visually look at and bend the rubber connectors and see if there is cracks in the rubber.

Note that the tiny plastic nipples that are on the main vacuum line snap off easily. Merely leaning on the line can break them (that happend to me during an Oil Change).

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84 300D, 82 Volvo 244Gl Diesel