Have you ever wondered how a water pump promotes cooling, but still allows rapid engine warmup on a cold start? The Chev water pump has a complex thermostat, designed with two gates. Chev engineers unconventionally chose to mount the thermostat on the radiator outlet hose as it returns cooled coolant to the pump, but it senses fluid temperature on the engine side near the bypass hole. The main part of the thermostat opens when hot and closes when cold, directly controlling flow through the radiator conventionally. However in addition, at the end of the themostat is a second valve that controls bypass flow. That bypass valve opens when cold and closes when hot, directing flow quickly back to the engine for more efficient warmup on cold days. By having a second bypass gate, when the engine is hot the gate prevents any fluid from getting back to the engine without routing through the radiator first. If that bypass gate fails open or if an incorrect thermostat is installed, the engine will run chronically hot because the radiator is bypassed. This thermostat is said to be a 'reverse poppet' design because the pressure of cold water helps to keep the valve closed.
With a more complex design, thermostat malfunction can have more variance in presentation. If this engine tends to run chronically hot, it would be wise to inspect and replace the thermostat, especially if the system shows any evidence of pressure overload.
I dissect the water pump from a 5.3L 2000 Chev Vortec engine, with specific reference to how the weep hole acts as an early warning system and how the cooling system initially bypasses the radiator to allow rapid engine warmup.
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