The ability of the Toyota thermostats to open and close is due to the change in density and volume of the wax element. This will expand and contract at different temperatures, allowing the flange to allow or block the flow of coolant through a bypass tube. When the thermostat in your Toyota engine reaches a predetermined temperature, it opens up releasing the coolant that was in the engine so it can begin to flow out and into the radiator. By remaining closed until this temperature is reached is how the engine is capable of warming up faster so it can perform as expected by the owner of the Toyota. Because the only moving part in the Toyota thermostat is the metal spring, this is the only component that can cause this cooling system component to fail. It can fail either by remaining closed all the time or remaining open all the time. This being open can be fully open or just partially. In any event involving the thermostat as described above is the situation your car is in it will be negatively affecting the performance and mileage of your engine. If the valve remains closed in the thermostat, the engine will overheat in just a few minutes once it is started. If the valve remains open, the engine will not be able to reach its optimum temperature it was designed to perform at. In either situation the cooling component has to be replaced before the engine will be able to operate as it is expected to. If the Toyota owner has an overheating vehicle because of a thermostat that will not open, they can remove the hose at the housing and jam a screwdriver down through the valve. This will make it possible for the coolant to flow until a proper repair can be done to the vehicle.