6.5 diesel coolant loss

Check the connector from the water pump/thermostat housing to the heater hose (metal to swaged flex hose) that leads into the firewall. If it's dripping from that "quick connect" onto the intake manifold, it'll evaporate and not leave a trace on the ground.

The only other thing I could add (which is not likely in your situation since you don't mention any other problems beyond coolant loss) would be the head gasket between the rear cylinders. On early 6.5's that have been used for heavy hauling or towing (1993-1997 I think) there was burn through between the rear cylinders caused by excessive heat under load. This problem was fixed in '98+ with the use of a higher capacity water pump with dual thermostats and a crossover. If this is where your coolant is going then it's not an easy fix, but if you do have this issue and your truck is pre-97 then I strongly suggest installing the later style water pump and plumbing I mentioned above.

Cheers - Jonathan

Thanks for the replies - Its either the head or the gasket. It leaked down about 1 psi every ten minutes when my mech buddy pressurized the rad. There was water getting into the oil too but its harder to tell on a diesel. Oil still looked black.