The thermostat's job is to block the flow of coolant to the radiator until the engine has warmed up. When the engine is cold, no coolant flows through the engine. Once the engine reaches its operating temperature (generally about 200 degrees F, 95 degrees C), the thermostat opens. By letting the engine warm up as quickly as possible, the thermostat reduces engine wear, deposits and emissions. The secret of the thermostat lies in the small cylinder located on the engine-side of the device. This cylinder is filled with a wax that begins to melt at perhaps 180 degrees F (different thermostats open at different temperatures, but 180 F/82 C is a common temperature). A rod connected to the valve presses into this wax. When the wax melts, it expands significantly and pushes the rod out of the cylinder, opening the valve.

your radiator is large enough to cool your engine to the point that it is below operating temp which is not good for your engine. the thermostat opens and sends the coolant from inside the engine to the radiator. they are set to open at a certain temp. it is mechanical not electrical. it is simply a spring that holds a valve shut until the coolant heats up and builds enough pressure to open the valve and send the coolant to the radiator to be cooled. when these go bad they can cause the engine to cool then it is not up to temp. if it is stuck open, and your heater wont work because the engine is not running hot enough. or it will cause the engine to over heat because it is not sending the coolant to the radiator. it depends it if failed in the open position or the closed position.

Both good answers but there is a secondary job of the thermostat. It also holds the coolant in the radiator for cooling purposes. Older engines didn't necessarily need a thermostat, if one stuck you could take it out and run for months without it. Heater wouldn't work because the coolant never heated up. Today's higher reving engines will overheat rather quickly without holding the coolant in the radiator for cooling.

Perryman's answer is great. He's exactly right about the actual mechanism of the t-stat. I'm not sure what the heck 'fritomech' is talking about saying the t-stat holds the coolant in the radiator for cooling. Preposterous idea, the t-stat holds the coolant OUT of the radiator to keep the engine temp UP at operating temp. Engines run most efficiently at normal temperature, plus it keeps your heater working properly. If your engine never came up to operating temp, you'd get decreased performance, economy, and longevity. Plus you'd freeze in the winter. Thermostat opens just enough to keep the engine coolant at the prescribed temperature. You can actually buy a t-stat that opens warmer or colder, depending on your climate or preferences. Say, a 185 degree instead of a 195 degree.

A thermostat prevents coolant getting to the radiator until the engine warms up. Once the engine reaches its operating temperature, the thermostat heats up and the thermostat valve opens. The thermostat helps reduce engine wear and tear and excess emissions. http://www.10w40.com/individual/100608.asp

thermostat controls the temperature of coolant inside your engine.when at the correct temerature it will open letting that coolant go to the radiator to be cooled,while letting a new amount of coolant enter the engine for the same cycle.

Many years ago, my electrical instructor told the class that the best answer to such a question is "DAMN GOOD". What he was trying to break us of was asking indefinite questions, such as the above. He taught us the way to ask about the functioning of any device was to ask "WHAT ARE THE PRINCIPLES OF FUNCTION THAT OPERATES THIS DEVICE?" I worked at a plant many years ago where the company made thermostats. A thermostat contains no wax. The secret of thermostats is the expansion of of a cylinder which has been compressed so that it expands when heated. A valve is operated by the expanded compressed cylinder to allow water to flow out of the engine into the radiator, which cools the water. Go to a parts store and buy the thermmostat for your car to see how the thermostat is constructed. Look carefully at the thermostat, then try putting the thermostat in a pot of water and boil the water to a good rolling boil. Observe the expansion of the compressed cylinder and the opening of the valve. Let the water cool, then watch as the thermostat shrinks in length, and closes the valve. Don't worry about damaging the thermostat, they are made of copper alloy. The explanation is Wikipedia is wrong. There is no wax in an automotive thermostat. Lucien

ask a mechanic stupid head