Many years ago engine water pumps ran full time, pumping the water through the engine block but not through the radiator. Thermostats in the top of the radiator contained a spring-loaded valve, the spring being made of two dissimilar metals (bi-metallic, just like the ones in your home) and when the temperature got to 180o, the metals would expand, open up the valve, and permit the water pump to start pumping the water though the radiator where the onrushing air due to the vehicle