The cooling system on my daughter’s car is overheating. Fairly new water pump, new thermostat, system recently flushed. When the motor is running (idle or while driving), having the “defrost” button (turned on or off), seems to make a big difference in the engine cooling system. When defrost is on, idle is smooth, radiator fans running at idle, water pressure in system seems to be normal, temp gage normal, water hose from overflow reservoir and the hose leading from the engine to the radiator, seem to be the same temperature, pressure in the reservoir seems normal. At this point, simply turning the defrost button off, causes the water in the reservoir to fill up and increase pressure, water temperature rises, the temperature on the outside of the reservoir goes from 125 degrees to 160 degrees, the pressure release valve in cap blows, and the radiator fans quit running. By simply turning the defrost button on, the motor immediately starts to idle smoother, radiator fans start running, pressure in overflow reservoir is reduced, and engine temperature is reduced. Before discovering the effect of the defrost button being on or off, while trying to define the problem, we noticed (after a short drive), the hose from the reservoir to the engine, was quite hot, while the hose from the engine to the radiator, was much cooler. We loosened the clamp on the water hose from the engine to the radiator, and kept idling. The reservoir got hot, and started leaking (hose from engine to radiator, still cool). While idling, we turned the defroster On, and immediately, the hose blew off, a lot of coolant blew out. We reattached, the hose, and from then on, both hoses seem to remain about the same temperature. We started this problem solving process, because we were told by my daughter’s mechanic, that the heater coil needed to be replaced, and before starting that gigantic task, thought narrowing the problem down might help. Not sure if the “power” of the defrost button to regulate the coolant flow, confirms that the heater coil needs to be replaced, the radiator is plugged, or what. Any ideas?