The main suspect is going to be a leaking cylinder head gasket. The additional clue to add to the likelihood is your observation of no coolant under the thermostat. Thermostats open in response to hot liquid. They do not open in response to hot air. A leaking head gasket can allow combustion gases to leak into the cooling system and pool under the thermostat, causing it to not open. Your mechanic can do a chemical test at the radiator to verify a head gasket is leaking. That involves drawing air from the radiator while the engine is running, through a glass cylinder with two chambers partially-filled with a special dark blue liquid. If a head gasket is leaking, the liquid will turn bright yellow.
To not overlook something, did you bleed the air out after you replaced the thermostat? If not, that alone will cause it to not open and the engine will overheat.
Monday, January 27th, 2014 AT 10:25 AM