The thermostat has basically one purpose. For quicker engine warmup. Once it's open, it's open. You think it's gonna close again with the engine running? No, because it sees coolant above the marked temperature on the engine side of the thermostat the whole time the engine is running after warmup. All the thermostat does is regulate flow to warm the engine up. Period. It's the radiator's and all accompanying accessories such as the fan, shroud and water pump that KEEP the engine cool, NOT the thermostat.
