Flushing the system with biodiesel may or may not work as it may take time to dissolve deposits inside of the fuel injection pump and injectors.

Flushing the fuel system does nothing for sticking piston rings. Perhaps you need to dump something inside of the cylinders to soak and soften any stuff between the rings. When you are done crank the engine without the Glow Plugs or the injectors to get rid of any excess fluid pooled in the recess in the pistons.

Change the oil before running as what ever you dump into the cylinders is going to run passed the piston ring gaps and into the crankcase.

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84 300D, 82 Volvo 244Gl Diesel