Why does car overheat until thermostat opens then runs fine?

A car overheats until the thermostat opens because the thermostat regulates coolant flow between the engine and the radiator. When the engine temperature exceeds a certain threshold, the thermostat opens, allowing coolant to circulate and absorb heat. This process prevents the engine from reaching dangerously high temperatures. Once the thermostat is open, the engine can maintain a stable operating temperature, resulting in normal performance.