ugh, i hate when people say to Replace things first. Thats the worst thing you can do.. Your just wasting money on a crap-shoot that "MIGHT" fix the problem. Diagnose the problem. Not saying everyone said to just replace stuff, but I see it all the time!!!
1. Get your Coolant system Pressure checked. If you have an air compressor you can rent the gauges at Autozone or Advanced auto and you will get your money back when you return them. Set the regulator on the gauge to 15lbs, pop off your radiator cap and screw the gauge you rented into the top of the radiator. Put air to it and let it pressurize for about 20 - 30 minutes. Get yourself a shop light and check around everything that deals with coolant.
1a. make sure to check on the bottom of the motor as well. if its leaking through the weep hole it will leak down to the bottom of the motor and drip on the ground.
1b. Also make sure to get your Cap pressure checked as well, a leaky cap could cause your car to not maintain proper pressure and will overheat.
2. If you see no leaks, Pull your timing cover off and grab your timing belt and pull on it, looking at your waterpump to see if it moves. If it has play in it, your waterpump bearing is culprit and you should replace your waterpump.
3. If the thermostat is broken in the closed position the engine would overheat rapidly. If it was broken in the open position the engine cooling system would not reach normal operating temperature and the heater would produce only warm air. Take the thermostat out and put it in a pan of water on the stove. When the water gets to around 200 deg F, the thermostat should open. If it doesn't it needs replaced. Observe carefully that you have not installed the thermostat upsidedown.
One way to do this is to grab your upper radiator hose and your lower radiator hose and feel the difference while the motor is heating up. If you can feel the lower hose all of a sudden get hot, then your thermostat is working properly. If it stays cold or warm the whole time (noticeably different temperature compared to the upper) Your thermostat is probably stuck. *** NOTE: I AM NOT RESPONSIBLE FOR INJURY. THE COOLING FANS CAN TURN ON AT ANY SECOND AND THIS METHOD IS NOT RECOMMENDED FOR SAFELY DETERMINING A BAD THERMOSTAT (but it does work

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You can also pull the thermostat out, visually inspect it, boil water and drop the thermostat into the water. If it is working properly it will open up on its own. If it is stuck it will not open.
Try these steps and you will spend about 30 - 40.00 USD in renting but overall it will be free because you will get your $$ back when you return it