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.
Thursday, January 27th, 2011 AT 7:37 PM