I have a Pontiac Grand Am 2001 SE 6Cyl. 3.4 engine, a few months ago the car was overheating, so I take to the mechanic and he told me that the main gasket was bad, so it was changed and do some mayor repair to the engine heads, after that the car was running very fine, a few months ago, the car starts overheating again so i take it again to the mechanic, this time the cooling system was purged and cleaned, the thermostat was changed, the temperature sensor was changed and the water pump changed too, finally the heater hoses was disconnected and connected directly without passing the heater radiator. after all of this, the car is running fine and no overheating problems but there is something I can figure out and is that the cooling fluid is always gone after 1 day of use, there is no signal of leaks, I check the oil and there is no water there too, any idea or suggestion about this?