I am assuming you are using Diesel Fuel and not an alternative Fuel.

Random ideas:
Abnormally tight Valve Clearances could be causing the Valves to stay open slightly after they are hot.

While the Valve cover is off line up the Camshaft Gear and Camshaft Bearing tower timing marks with the Piston on the Compression Stroke and take a look at the degrees showing at the pointer on the Crankshaft Dampner. To see how far off your Camshaft timing might be due to Timing Chain Stretch and drive Gear wear.

Another guess something is interfering with your Fuel Delivery to the IP and/or injection of Fuel Into the Engine.
I would try pulling the shutoff Valve Vacuum Hose/Tubing and plug the Hose. To see if for some reason some Vacuum is getting into the system from that worn Vacuum Valve that is attached to the Steering Wheel Lock housing.

Trouble shoot for air leaks.

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84 300D, 82 Volvo 244Gl Diesel