Things that effect the idle.
Valve adjustment
Injector Nozzles worn of plugged with Carbon
Motor Mounts
Transmission Mount
Air Leaks in inlet Hoses or crack in the Plastic Filter
Air leaking from Old style Hand Primer that has the shiny Aluminum Body
Fuel Pressure Relief/Overflow Valve not performing properly
Rack Dampener Bolt needs adjusting or Updated Gold Colored one
Idle speed set within the limits that are on your emission Sticker.
The Engine Shock Mounts being rotted out
The Fuel Supply/Lift Pump needs rebuilding

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84 300D, 82 Volvo 244Gl Diesel