It could be the water pump, hoses, thermostat, a clogged radiator, or air in the system that needs to be bled out.
I'm voting air.(But i could be wrong)
Has the coolant ever gotten real low? If so it could be air. There is a bleed screw on the thermastat housing.
Check out this website for good info from Mazda Tech Manuals + pretty pictures with locations
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It is recommended to change engine coolant every 30,000 miles. The refilling has to be properly done in order to avoid the introduction of air into the system. The correct procedure is as follows (assuming the radiator drain plug and engine drain plug are in place):
* place heater control to maximum heat
* remove the radiator bleeder plug
* remove filler cap
* add 50/50 mixture of ethylene glycol and (distilled) water into the filler neck (where the filler cap screws in)
* once fluid starts coming out through the radiator bleeder hole, go into a "add and hold" mode (add a little and then wait a little) until fluid comes out as soon as you put it in
* screw in radiator bleeder plug
* fill the reservoir with coolant
* start engine and let it run until the thermostat opens (upper radiator hose is hot to the touch)
* add coolant (as necessary) into the filler neck
* screw in filler cap
Good Luck!
-Project#7