What do you do when your car runs out of water?

Fill the vehicle with water, start the motor. As the vehicle warms up you should be able to see where the water is leaking out of the motor. Look at the radiator, water pump and the hoses. If you don't see any water leaks then your problem could be the heater core is leaking or you have either a cracked head or a bad head gasket. If this is the case, look at the underside of your oil fill cap. If it is milky brown that is either the head or the head gasket.