Ok some background info first. Replaced radiator, waterpump, head gaskets,
coolant tank and cap, thermostat, heater control valve and hoses, flushed heater core bi directionally. Heater works then doesnt. Sometimes works at idle sometimes moving down the hwy. Just kind of comes and goes with no rime or reason. Checked coolant level bled coolant system as described in threads. Temp gauge stays right in the middle. Checked belt routing as per diagram. The only odd thing I have noticed is the PO pinned the fan clutch so that it is locked all the time and the radiator feels cool to the touch never hot.

Any Suggestions?

 

Replace the fan clutch, if it is running all the time the truck is running to cold.
Check the tech section for a more affordable replacement.

 

I would say the control valve or air in the cooling system. If the fan was cooling too much, the thermostat should just close and heat things up

 

Thus the heat then no heat situation.

T-stat opens, coolant cools to much, no heat.
T-stat closes, coolant heats back up, heat.