This is in general.

Loss of Power causes and trouble shooting
Starting with the most frequent cause/s remember there can be more then one cause.
Fuel Filters restricted- Change Fuel Filters.

Fuel Tank Screen- Remove and clean Screen or replace it.

One way to trouble shoot. Swap the positions of the Fuel Return hose with the fuel inlet hose and be sure you have 1/2 Tank of Fuel. That reverses the flow in the hoses and does not pull fuel through the tank screen but from a higher position in the Tank (which also means you would run out of fuel before the gauge shows that). If you power is restored when you do that your Tank Screen is restricted.

Plugged or restricted Fuel Tank Vent. Mud and road gunk can plug the outlet. On a WQ123 it is in front of the left rear fender.
One way to troubleshoot- Have about 1/2 tank of fuel (you don't want a full tank) and drive without the Fuel Fill Cap. Having no cap acts as the tank vent. If it drives normally with the Cap off your vent is likely plugged up.

Fuel Pressure Relief/Overflow Valve- If it is the type that can be dissembled the spring in it can be stretched back to 27mm of free length. Otherwise it needs to be replaced

Fuel Supply/Lift Pump- The Valves inside can get gunked up and the Piston can stick. There is rebuild kits for it or you can take it apart and clean it out.

Throttle linkages from the Pedal to the Fuel Injection Pump out of adjustment or parts worn or broken.

Plugged Air Filter or air inlet system.

Startron for diesels as has an enzyme that eats up stuff that grows in diesel fuel and breaks them up into small enough pieces they pass thorugh the Fuel Filters.

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84 300D, 82 Volvo 244Gl Diesel