Was the vehicle run on some alternative fuel like Waste Vegetable Oil or?

Inside of the Fuel Injection Pomp the individual Elements (plungers and barrels) has a small feed hole or holes. Even old dried diesel fuel can plug them up.

Need to know if you have an MW or an M Fuel Injection Pump.

Please note if your vehicle has an MW Fuel Injection Pump at the top of the pump there is 10 nuts that a 13mm wrench will fit. Don't loosen those nuts or you will change the amount of fuel each element puts out. And, the pump would need to be recalibrated.

So the mentioned fuel feed holes on the elements can be plugged.

Even with clean diesel I have seen Fuel Injection Pumps that sat in a shop get plugged and also I have seen when the Fuel Injection pump is rotated the plungers get stuck in all the way up and don't come back down to so they could get filled with fuel. And, that is in a shop clean fuel and on a shelf.
With fuel from the thank it is possible for there to be water and the plunger go up and stick from a combination of rust and dried up gunky Fuel.

Sticking rack: If the plungers in the elements or ever one element is stuck the rack won't move. The other way a rack won't move is if there was lots of sticky gunky oil that dried up inside of it.

If you remover the vacuum shutoff you will find it hooks onto the rack and you can see if the rack moves back and forth. Look up re-installing the shutoff because if you stick it in the wrong slot the engine runs away and you have to block off the air or stop the fuel supply as the shutoff is not going to work.

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84 300D, 82 Volvo 244Gl Diesel