What Does a Car Thermostat Do?

Just like a house, a car has a thermostat. Engines need to operate at certain temperatures so that they can run reliably and efficiently. Most car’s coolant operates in the 195°F to 220°F temperature range. Its very important for engine health that the car’s coolant is not too hot or too cold. The thermostat plays an important role in the coolant system. This article will discuss what a thermostat is, how it works, and it’s function in the cooling system.

What is an Engine’s Thermostat?

Engines are extremely hot. In fact some internal combustion engines can reach over 4,000°F inside the combustion chamber of the engine. That is why it is important to have this heat transferred to the coolant where it can be cooled off in the radiator. The thermostat regulates the engine coolants temperature in the 195°F to 220°F range.

A car’s thermostat is basically a valve that is located in the cooling system between the engine and the radiator. Its only purpose is to govern the amount of flow coming from the engine to the radiator. When the thermostat is closed, it blocks coolant from flowing from the engine to the radiator, thus increasing coolant temperature. When it opens, hot coolant rushes into the radiator and the heat from the coolant is dissipated.

A car thermostat decides to let coolant flow from the engine to the radiator based on the engine’s temperature. When the engine is just hot enough, the valve in the thermostat will open up and allow the coolant to cold. Cold engines will have a closed thermostat if it is functioning properly.

Above is an example of a typical thermostat

Car Thermostat as a Component of the Cooling System

The thermostat is an important part of any vehicle’s coolant system. If vehicles did not come equipped with thermostats, the engine would struggle to reach a proper temperature. This would cause the coolant to immediately be cooled by the radiator greatly reducing fuel economy. The reason fuel economy would suffer with no thermostat or even a stuck open thermostat is that the ECU generally gives the engine a rich fuel mixture when its cold. The extra fuel would normally heat the engine to operating temperature, but instead it reduces fuel economy and the engine remains cold. On the other hand a closed thermostat will not allow the coolant to cool off overheating the engine. Overheating can severely damage the engine.

How it Works

While the function of the thermostat is simple, how does it know when to open or close? Glad you asked! The thermostat has a wax cylinder that is placed into the engine side of the cooling system. This wax cylinder is connect to a rod which connects to the thermostat valve. As temperatures rise, the wax cylinder will expand. The expansion of the cylinder pushes the rod upwards. As the rod moves upwards, the valve attached to the rod also moves upwards. With the valve now open, coolant flows to the radiator. Since this is a purely mechanical function, the hotter the coolant gets, the further the valve will open. A larger area will allow more coolant to flow out of the engine and into the radiator. As the average coolant temperature drops, the valve will slowly close.

During normal operation, the thermostat is neither completely closed or completely opened. Normally the thermostat is slightly opened allowing a specific amount of coolant to be cooled. A proper functioning cooling system will keep the dial on your temperature gauge steady. Depending on the engine load the thermostat will either open up further or close up. A truck traveling uphill will cause the thermostat to open further as there is increased load and more heat produced by the engine.

Because the thermostat can keep your vehicle from overheating or under-heating, it plays a critical role in the health and longevity of your engine.

Symptoms of a Bad Car Thermostat

Knowing how a thermostat works is very important to troubleshooting issues with your vehicles cooling system. Most of these symptoms we’ve already covered above, but we’ll briefly overview a few issues here.

Engine Overheating – If the thermostat is stuck closed or is not opening far enough, your engine is going to overheat. The coolant is going to remain trapped in the engine where the hot engine is going to “cook” the coolant. This is a serious issue and you shouldn’t drive your vehicle if the temperature gauge shows that the vehicle is overheating. If you have issues with engine overheating, changing the thermostat can be the solution. Other possible causes include low coolant, a failing water pump, or a damaged radiator.

Engine Under-heating – Like overheating, this is another serious problem. If the dial on the temperature gauge never makes it into the low end of the operating range on the gauge, you might have a stuck open thermostat. Apart from a low temp reading on the dial, you could notice reduced fuel efficiency and lack of heat when you turn on the heater. Engine under heating can also be caused by leakage around the thermostat. If the sealing gasket is worn, the coolant will follow the leak path through the valve. If enough coolant leaks through it can prohibit the engine from heating up.

Fluctuations in Temperature – If the engine temperature is fluctuating, this is usually the result of a malfunctioning thermostat. The thermostat is not staying still in a single position it is either closing or opening too much causing variation in temperature readings. If you are experiencing fluctuations in engine temperature we recommend replacing the thermostat especially if you know how to replace it yourself.

How to Test a Thermostat

Checking to see if your thermostat is working is fairly easy. Open the hood of your vehicle and remove the filler cap on the radiator. Start the car and let it idle. You should not see any coolant flowing into the radiator as the engine is still cold. If you do see coolant, the thermostat is opened when it should not be. If you do not see any coolant flowing into the radiator, continue idling the vehicle until it reaches operating temperature. As the engine warms up the thermostat should open allowing coolant to flow into the radiator. If you do not see coolant flowing into the radiator and the temperature gauge continues rising, you probably have a thermostat that is stuck closed. If the thermostat is stuck either open or closed, it should be replaced.