Loss of Power on Diesel Engines if often restricted or poor performing Fuel Supply System.

Changing the Fuel Filters is the first place to start.

It also might be that his Throttle Linkages have become Sloppy over time and the Throttle lever is not being pushed far enough. On my 123 part of the Throttle (Control) system is a Bell Crank on the Fire Wall. It pivots on a Plastic Ball behind the Bracket.

That Ball falls apart with age and the front portion of the Bell crank has a lamination of metal and Rubber that rotates the Throttle Rod. If the Rubber rots out the Rod is not going to mover properly.

Thin linkage you are speaking of may not be the same as mine but a Visual Inspection of the Linkages would be a resonabel thing to do.

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84 300D, 82 Volvo 244Gl Diesel