The coolant fitting at the back of the head has a thick o-ring. There's a circlip holding the coolant line to the head. Along the circumference of the circlip there's a hoop you can hook with a pick to pull off the circlip. Then the line will pull out of the head so you can replace the o-ring. It's the same o-ring as used in the heat exchanger line attached behind the water pump, described as water pump o-ring in FastLane. You can check with Phil to be sure.

It's difficult to see a water pump leak in these engines.

Sounds like it's time for new fuel filters and maybe a dose of Diesel Purge.

Sixto
93 300SD 3.0