AR_Cobra Did you add a couple of 1/8" air bleed holes in the thermostat before installing? Some guys raise the front ends of the cars to get all the sir to the front of the motor. 50/50 mix of
antifreeze and disstilled water, and 1 bottle of water wetter. depending on what part of the country you are in. You could try and vacuum the system down first and then refill again. The other issue I have is did this problem just start??
Other possiblitys for problem are impeller spinning free on shaft, when cold you have enough flow, when hot impeller may slip. This is a long shot.
Head gaskets starting to fail. Small pin hole leak causing compression into system. How long ago was motor built? How hard do you drive the car?? What was the
oil temp on the motor? Check the
oil and look at it for a discolor of brown. I will stop here and hope it's just air bound and will take a couple of cycles to remove the air from the system. You did check the thermostat housing for the same temp reading on it?? Have seen them boxed wrong unless bubble wrapped. This will make the motor run hotter or colder. The missing coolant is a different thing. Start with redoing thermostat and recheck numbers and follow beginning of ideas, then write back. Take readings all around the motor and radiator also. Simple check is to boil water and make sure the thermostat is opening on the correct temp. Rick L. Ps if these idea don't work, this could get ugly. Question left out, Are you hard driving of the car??Miss shifts? Racing C.S.? or other friends? You may have lifted a head with a missed shift. Car will normally over heat after 1/2 -1 hour of running. Last thing is to check pressure in coolant system during ride. You will have to rig up a gauge of coolant pressure tester to see reading. Good luck.