6.5 diesel running hot

If you find that it IS hot, check the upper radiator hose when running by squeezing it. If it's hard - i.e. gettting pressurized - you may have a head-gasket and/or cracked head problem. The 6.5s crack stock heads pretty easy. The new heads being made now (in China) are a huge improvement over the stock heads - much thicker castings, hardened valve seat inserts, etc.

Is that the one you replaced?

Are you SURE you got the correct part? A friend just went through something similar with a 1994 and wound up returning the "parts store" heat sender and ordering one from the GM dealer by "vin" number.

The other thing to check that will cause ALL SORTS of strange problems is that you have ALL the ground wires connected to the engine.

IIRC there may be some ground leads at the back of one or both cylinder heads and on several intake manifold bolts/studs. These ground leads may extend from the wiring harness or be braided ground leads from the metal of the cab.

I will check the ground straps. Come to think of it, I remember seeing a small braided cable hanging on the right side.

Thanks for the tips!

Make sure ALL air has been purged from the engine.

Does your's have the air vent screw in the thermostat housing?