My pt criuser has heat one min and then loses it the next and now its the temp gauge says its over heating but I dont see any signs that it is and im not losing any antifreeze and dont smell any! Changed the thermostat and didnt help! Please help!Also the fan does kick and there is no heat at all when the car in running hot!
Since you observed the coolant isn't low, suspect a leaking head gasket. Combustion gases will go into the coolant and bubble out the reservoir making it look like it's boiling. The clue is there won't be any steam with the bubbling.
The combustion gases can form air pockets in the cooling system that close the thermostat. Thermostats open in response to hot liquid. They will not open in response to hot air. Air pockets will also stop the coolant flow through the heater core.
Your mechanic can perform a "leak sniffer" test to confirm a leaking head gasket. That involves drawing air from the radiator through a glass cylinder with two chambers partially filled with dark blue liquid. If combustion gases are present, the liquid will turn bright yellow.