Things you must know about automobile thermostat

Just as the cooling system is very important in internal combustion engines, thermostat plays a very vital role. Keeping the engine at optimum temperature, a thermostat must be in good condition. With this component, the engine is preserved from getting too hot which could cause irreversible damage to the engine.

Car thermostat is located between the engine and the radiator. when it gets spoilt the complete cooling system of the engine will start malfunctioning, which could be severe. Since this component is a new feature in the engine cooling system most vehicle users find it difficult to understand how their engine is cooled.

automobile thermostat

Today you’ll get to know the definition, functions, parts, working, advantages and disadvantages as well as symptoms of a bad thermostat in automobile engines.

Read more: Understanding Cooling System in Internal Combustion Engines


Thermostat Definition

A thermostat is a temperature-sensing device that measures the temperature of engine coolant. The component is designed to know when to open and close so that internal combustion engines can work at an efficient temperature.

Thermostats remain close if the coolant is not hot enough. But if the coolant reaches a specific temperature, it opens, allowing the flow of hot coolant into the radiator. So, the thermostat is just like a gate that allows or blocks the flow of coolant from the engine to the radiator.

Modern car engines work at a certain range of temperature, normally, the engine work between 194-degree Fahrenheit which is 90 degrees Celsius and 221-degree Fahrenheit. The thermostat senses the coolant temperature and determined when to open and close.

Functions of Thermostat

Below are the major functions of thermostat used in the engine cooling system:

  • The primary function of a thermostat is to control the working temperature of the engine.
  • With the component, the engine is prevented from overheating and excess cooling.
  • At cold start, thermostat blocks the coolant to flow. This cause the engine to reach its working temperature faster.
  • Also, it ensures coolant absorbs heat before entering the radiator.
  • Finally, thermostat controls the circulation of coolant in the overall engine

Components of thermostat comprise wax, rod, cylinder, and valve. Functions of these parts will be explained in the working.

Thermostat diagram:

thermostat components

Read more: Components of internal combustion engine

Working Principle

To most newbie in the auto industry, the working of a thermostat can be so magical. But the fact that is less complex and can be easily understood. Just as earlier mentioned the components are located between the engine and radiator in the internal parts. So, from the radiator coolant passes through the down hose of it to the engine. This where the coolant absorbs heat.

However, the engine also has to work at a specific temperature so efficient working can be achieved. This is why the thermostat is needed, just imagine there is no thermostat in an engine and coolant flows to absorb heat produce. The vehicle might event stop working since the heat from the combustion process is taken by the coolant. But, if there is a thermostat, the flow of the coolant is blocked until the coolant get heated to some specific temperature where the device needs to open. With this engine work at the right temperature and will not overheat as well as undercool.

So, the big question on your mind is,

How does the thermostat open and close?

You can easily test a thermostat of an engine to see clearly how it fully works. You’ll need to get a thermostat in an auto part store for the testing. Place the thermostat in a pot of boiling water on the stove. As soon as it heats up, the wax gets melt and the valve open.

As earlier explained the thermostat consist of a cylinder, this cylinder is filled with wax. The cylinder which is a metal part lies inside the engine so it can come in contact with hot coolant. Inside the cylinder, there is also a spring valve that connects to a rod.

This is how it works, the wax is a combustible material and that can melt. So, when a cold coolant flows to the thermostat cylinder, nothing happens to wax, leaving the valve down to block the passage, but as soon as the coolant reaches a specific temperature and the cylinder senses it, the wax begins to melt slowly.

The melting wax is what pushes the rod outward movement. This rod helps to open the cylinder valve which then gives space for the hot coolant to flow. This is why the thermostat remains close at cold starting.

Read more: Things you need to Know about Automobile Radiators

Watch video to learn more on how thermostat works: 

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Symptoms of Bad and Failing Thermostat

Symptoms of bad or failing thermostat can occur in two-phase which include:

Thermostat Stuck Open:

When a thermostat is stuck open, the temperature of the engine extremely drops below the normal working temperature. on cold weather, the engine can even stop working, since the combustion is interrupted by the coolant. Thus, resulting in a lack of heat in the system. The coolant is expected to draw at least 30% of the heat production. So, with the thermostat stuck open, coolant has power over the combustion heat. This situation will cause the engine light to come on.

As underheating occur, lower working efficiency will occur in the engine oil. This can hasten the speed at which engine parts wear down. The lower working efficiency of the engine can also lead to poor fuel economy, which can also result in greater emission over time.

Thermostat Stuck Close:

When the thermostat is stuck closed, overheating might occur. As the device is stuck close the little coolant in the engine are stopped from flowing when heated. This will cause cooled coolant from flowing in. although there are some other factors that cause overheating, it could be because of a low coolant level in the engine or a failing water pump. Leaks in the cooling system can also cause the same problem of a stuck close thermostat.


In some situation where is the thermostat is overworked, some issues might occur like housing leaking, seizing, etc. This can result in fluctuations in engine temperature and poor engine performance. In this condition experience, the driver often gets confused with what goes wrong.

The temperature of the engine can sometimes overheat and will eventually underheat. the thermostat tends to open or close when it’s not supposed to. This happens because the thermostat doesn’t get stuck in a single position, resulting in wrong temperature readings.

Since thermostat ensures engines to work at optimum temperature, it malfunctioning will lead the opposition. Thus, leading to problems in engine performance. One of the common problems with this is the poor fuel economy, and the engine will be hard-pressed to operate.

Read more: Understanding Supercharger in Automobile Engines

How to Test a thermostat

Testing a thermostat can be done without even removing the component to check. Modern cars thermostat is monitored by the engine computer (PCM). if it gets stuck open or closed, the check engine light will turn on.

Most mechanics test thermostat by measuring the temperature of the upper and lower radiator hose. This can be carried out with an infrared thermometer, which helps to monitor engines temperature.

In conclusion, a thermostat is a device with great advantages on the vehicles cooling system. The primary function is to control the flow of coolant, keeping the engine at the optimum temperature. in its working, the flow of coolant is blocked till it reaches the temperature where the thermostat opens. it can get fault being stuck either open or close.

That’s it for this article, I hope you enjoyed the reading, if so, kindly check some other interesting post on engines. Don’t forget to share and ask questions via our comment box. Thanks!

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