You need to give a little more info.
Check the water pump weep holes which are at the top and bottom of the pump behind the pulley. If they are leaking you have a bad pump or maybe your thermostat isn't seated properly in the housing.
Did you install the thermostat the right way? If it's backwards then that could be the problem =P
Is your coolant running out of the overflow tank? If so check to make sure the tube that runs to the overflow isn't pinched. That would cause it not to be able to get back into the radiator when it cools down thus causing the bottle to keep getting filled until it runs over.
Do you see hard water build up inside the radiator when you look into the filler neck? If so get a new radiator (recommended anyway if it's very old). Pep Boys has a heavy duty radiator for around $100 which works great for me.
Is there signs of oil in the coolant or coolant in the oil (milky brown colored substance)? If so, I'm sorry to say you either have a cracked or warped head or a blown head or intake gasket. Either way you have some work ahead of you.
What kind of fan are you running? If it's an original clutch fan does the clutch feel like it still has some friction left in it? Auto Zone has electric fans pretty cheap. I got the largest one they had and have no problems with it even in stop and go traffic. Cost me $65.
Do you see any noticable leaks?
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Too much to list. Best ET on the car so far with stock 2.73 gears - 13.73@102mph
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