That's what the thermostat's job. Slowing or virtually stopping the flow of water till it reaches the temp printed on the thermostat itself. Once that temp is reached, it opens and lets the water flow through the engine to the radiator to be cooled, but when the temp of the water gets too cool, thermostat partially closes and slows the water to heat up again. If the engine runs too cool, it loses power and gets shyte fuel economy. Happened to my first 260Z; when I replaced the thermostat, was like chalk and cheese!