i had the same problem, i hit a big rock and it put a hair line split in the bottom of my rediator and i couldnt notice the water leaking out, the water didnt leak out untill the system got alot of pressure on it, so i never noticed it until my car started over heating...sucks, check for leakage, get on some pavement and turn ur car on and let it idle for a while and let the pressure build up in the system and check for any leaks under the car..