I have not read of the Injector Fuel Return Lines causing a rough Idle.

My first thought would be an Air Leak, if you still have that old style Hand Primer with the shiny Aluminum body on it that would be the first suspect. After that for a few dollars you can change the 2 Fuel inlet Hoses with 5/16" Fuel Hose from the local Auto Parts store (it is rated for Diesel Fuel); changing the Plastic Primary Filter will also eliminate it as a source of an Air leak.

After that I would be thinking Injectors and/or Valve adjustment.

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84 300D, 82 Volvo 244Gl Diesel