Coolant overfills into reservoir. Then spills. Then the car overheats. Heater works while it over heats; just not as hot as it should be. Changed thermostat. Changed radiator cap. Doesn't look like my pump is leaking or out! (When the pump leaks it means it's out right?) No cracks or any other leaks anywhere that I can find. Although radiator cap does may a very low hissing noise that you have to put your ear to the radiator cap to hear. Please help!