Loss of power on Diesels first thing to do is change the Fuel Filters and bleed the air out of the fuel supply system. Look at the plastic fuel filter and see if it is clear enough for you to seen inside of it. A lot of black gray particles inside mean that you likely have something growing inside of the fuel tank and that is what is causing the filters to plug up.

There is also a fuel tank screen that can get plugged. The check for the fuel tank screen is to have 1/2 tank of fuel and swap the positions of the fuel return hose and the fuel inlet hose where they come off of the tubings at the fender well.

As mentioned the vacuum pump issue can be an engine killer. You will have to start and idle the engine but probe around with a Mechanics Stethoscope or my favorite a piece of 2 foot long broom stick with my ear on the other end and see if you can locate where the sound is coming from. If it is coming from the vacuum pump don't run it any more till the vacuum pump is fixed.
Some people have changed the vacuum pump ball bearing so you may or may not need a new pump if the bering is no good.
You also need to inspect the timer gear face for wear when the vacuum pump is out.

If the fuel tank drain is plugged you will pull a vacuum inside of the fuel thank. If you remove the fuel fill cap the open tank fill tube will become the tank vent. Meaning if that cures the issue you know the tank vent is the problem. However, people have said if you drive like that with a full tank fuel comes out of the fill tube.

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84 300D, 82 Volvo 244Gl Diesel