How Does A Car Thermostat Work?

The average human body sits at about 98.6°F, if it’s just a few degrees higher, it suggests something is off. A car engine works much the same way, averaging between 195-220°F. One of the main components in charge of regulating how hot or cold it gets is the thermostat.

How does a car thermostat work?

Simply put, it responds to temperature changes in the engine coolant. If it is cold, the thermostat stays closed, keeping it in the engine. If it gets too hot, the thermostat opens, allowing it to flow to the radiator where it will cool down.

Still not sure? No worries, because in this article, we’ll cover everything you need to know about the automotive thermostat. Including what it is, how it works, and how to tell if it’s not operating correctly.

Finally, we will provide a step-by-step guide explaining how to check a car thermostat.

What Is A Car Thermostat?

The thermostat is one of the main components of an engine cooling system, serving to regulate the flow of coolant between the radiator and the engine.

While it may be small in size (fits in the palm of your hand), the job it performs is crucial for your engine to stay within a safe operating temperature.

Too much heat for too long, and you can end up with a cracked engine block or blown head gasket (basically the two worst-case scenarios).

How Does A Car Thermostat Work?

As coolant flows through the engine, it picks up excess heat. Once it leaves the engine, it goes to the radiator, where that excess heat is removed. It then makes a few more stops along its way through the cooling system and ends up back in the engine.

The thermostat is basically a valve between the engine and the radiator.

When the coolant in the engine gets too hot, this valve opens, letting it pass to the radiator where it will cool down. If it is not heated, the thermostat stays closed, allowing the coolant to continue circulating inside the block.

How does the thermostat know when to open or close? It features a unique type of wax that acts as an expanding agent. When the heat reaches a specific temperature, the agent expands, which opens the valve.

When the heat subsides, the agent shrinks, returning the valve to its usual closed position.

As you might have guessed, an engine that’s running on hot coolant won’t be able to cool down, meaning it’s at risk of overheating. Let’s now look at a few of the most common signs that suggest your thermostat needs attention.

What Are The Symptoms Of A Faulty Thermostat?

Poor Engine Performance

Imagine going out for a long hike on a hot summer day only to find out you didn’t bring any water. There’s no way you could continue for long.

This is precisely how it works for your car too. If the thermostat is not functioning correctly, the engine has no way to cool itself down. When this happens, you’ll notice a drop in engine performance as it struggles to keep up.

One of the more obvious signs that something inside your cooling system is malfunctioning is a drop in fuel economy.

Engine Overheating/Underheating

As one of the primary devices in charge of controlling the temp inside the engine, if the thermostat is not working, the gauge will end up outside the range it should be.

If the thermostat is stuck in the open position, the coolant will continually flow. This can hinder the engine from reaching its optimal operating temp, reducing things like engine power and fuel economy.

The other side of the equation is an engine that’s overheating, which can be a severe issue. This happens when the thermostat gets stuck closed, not allowing hot coolant to cool down in the radiator. When this goes untreated for too long, it can cause real harm to your engine.

While many things can cause an engine to overheat, a faulty thermostat is one of the first things you should check.

Fluctuating Engine Temperature

Another temperature-related symptom is fluctuations in the gauge. If you notice it going from hot to cold (or vice-versa) frequently, it may be due to a faulty thermostat.

If the thermostat is not opening/closing as it should, the coolant flowing out of the engine can’t be regulated. This can confuse the system and cause it to display incorrect readings.

Coolant Leaks

Another sign that your thermostat is malfunctioning is if you see orange or green puddles of liquid under your car. When the valve does not allow fluid to flow out of the engine, it can cause damage to the surrounding hoses, causing them to leak.

If this symptom matches with any of the others on this list, it may be time to check the thermostat.

How Do You Check A Thermostat?

Thankfully, checking a thermostat is pretty straight-forward.

First, start the engine and let it idle for a minute or two. Then, find and remove the radiator cap, and look inside to see if there’s any coolant flowing.

By this point, the engine shouldn’t be hot enough for coolant to be necessary, so it shouldn’t be flowing. If it is, the valve is likely stuck open.

The other possibility is that the valve is stuck closed.

To check for this, let the engine warm-up until it’s reached its optimal operating temperature, then look to see if any coolant is flowing. If it’s not, it should match with a rising temperature gauge shown on the instrument panel.

Either scenario suggests you need a new thermostat. Depending on how savvy you are under the hood of a car, you may be able to replace it yourself for between $50-$150. Otherwise, a shop will handle it for you for between $200-$300.

With Small Size Comes Great Responsibility

Size aside, the thermostat in your car has a vital job to perform. If it’s not working as it should and you ignore the symptoms that suggest so, you’ll eventually end up with a massive repair bill (which does matter).

Be the responsible car owner and take care of it throughout its lifetime. This is the absolute best way to ensure it stays running strong.

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Joshua Barrett

Josh Barrett is a writer hailing from the great state of Alaska. While describing himself in the third person is not his forte, writing about any and all things automotive – is. After 13+ years hustling in the exciting world of car sales, he took off to travel the world with his dog Teemo.