If it really is leaking between the lower Injector Nut and the upper body you can remove the Injector and torque it again (I do not know the torque). You if you have enough room and a Large Enough Open end wrench you can do the same without removing the Injector by holding the lower Injector Nut with the Open End Wrench.

I think the older Engines use a Copper Crush Washer to seal the end of the Injector to the Prechamber. The make Copper and Aluminum Crush Washer replacements.

If torqueing it again does not solve the problem it could be that the lower Injector nut has developed a hair line crack. It also could be that the Injector will have to come out of the Engine and the Injector taken apart and the interior flate surfaces lapped flat again before it is reassembled.

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84 300D, 82 Volvo 244Gl Diesel