Cooling System design

One big difference between cooling systems today and 50+ years ago is the use of a double-poppet thermostat on the pump inlet (instead of a single-poppet on the engine-out). This requires the bypass circuit, which includes the heater core, to always be flowing, except when the coolant is very hot and the stat is full-open. If your stat sticks in the full-open position, you will have no heater core flow because the second poppet blocks the bypass. It is designed this way to maximize the coolant flow when you need maximum flow through the radiator.