Cooling problem

[jpmartinson]

Hi everyone,
I am having the same problem with two different trucks. One is my 71 GMC 3/4 ton 4x4..

When I start it up, it slowly heats up to the thermostat opening temp, the thermostat opens and the temp drops and stays cool/cold and never heats up again. When driven, temp slowly goes up and just keeps going up. Acts like a stuck thermostat. Have replaced the thermostat a couple times, even put it in some hot water on the stove with a thermometer and verified that it works properly but the problem still persists. New fan clutch, new water pump, new radiator, new temp sender and new cap. Anyone have any ideas?

Thanks for any help.