If you have recently changed the thermostat, it is likely that an air pocket is trapped in the system. There are air bleed nipples on or near the thermostat housing on many Japanese engines. If your engine has such a bleed nipple; when the engine is cold, open the nipple by turning it conterclockwise 1/2 to 3/4 turn. Then remove the radiator cap, and pour a mixture of 50% antifreeze and 50% distilled water into the radiator. Air and antifreeze will begin coming out of the nipple while you are pouring. Keep pouring fluid into the radiator until the air stops coming out; and there is a solid stream of antifreeze coming from the nipple. Then shut the nipple, and top up the radiator. If there is no bleed nipple on your engine, you can do the same thing by removing a temperature sensor or any threaded object which screws in near the highest point of the cooling system. As a last resort, you could loosen the thermostat housing to allow air to bleed out.
If it is possible that the thermostat could have been installed upside down; that would definitely cause this problem. The spring on the thermostat should be on the engine side of the housing. And if you used 100% antifreeze, it would also cause this problem. Antifreeze must have at least 30% water mixed with it; in order to conduct heat properly.
If your head gasket is blown; you'd probably be either getting lots of white smoke out the exhaust (a small amount of white smoke (steam) is normal in cold weather; but a blown head gasket will create MUCH MORE steam than that); or the engine oil will look milky and have bubbles in it; or there will be a strong odor of exhaust from the radiator when you open the radiator cap.
A blown head gasket will begin forcing coolant out of the reservoir within a minute or two after starting a cold engine. An air pocket will take much longer before the engine begins to overheat. If your engine only overheats and boils the coolant after 5 or more miles of driving, it is likely that the electric radiator fan has stopped working. The fan should begin running whenever the temperature gauge goes much over halfway up. Electric fan failures usually are caused by a defective thermal fan switch.
I hope this helps!!!