A car thermostat is a small, temperature-controlled valve. Most vehicle thermostats are only 2 inches (5 cm) in size. But it is an important part of your car engine cooling system.
In this article, you will see what a thermostat does in a car. We also explain its proper working process and common symptoms of a bad thermostat as well.
Function of Thermostat in Car and Its Importance
You can find a thermostat in almost every liquid-cooled piston engine. The main job of a thermostat is to keep your vehicle engine at the right operating temperature.
Controls Coolant Flow
A traditional thermostat uses a wax pellet, spring and valve mechanism to control coolant (antifreeze) flow through the engine cooling system. It manages the path for coolant between the engine and radiator. This tool also acts as a temperature-sensitive valve in the cooling circuit and controls the amount of coolant that can pass through at a given time.
Fast Engine Warm-Up
The engine needs to reach the correct operating temperature, so it can maintain the proper stoichiometric fuel mixture. A stoichiometric mixture gives the engine the correct balance of fuel and air for efficient combustion.
Thermostat in your car helps the engine to reach its normal operating temperature quickly after a cold start. It limits coolant circulation through the radiator during the warm-up period. This allows the engine coolant to heat up faster.
Prevents Overheating
A car thermostat uses a wax pellet expansion mechanism to open and close according to the engine coolant heat. It increases coolant flow through the radiator if the engine reaches a high thermal level. This process allows the cooling system to remove excess heat.
Improves Fuel Efficiency
A properly warmed engine can burn the fuel more efficiently, which is possible due to thermostat. Hence, it reduces fuel waste during combustion. Moreover, the correct engine temperature keeps the oil at its right viscosity. This also decreases friction between moving parts and lowers energy loss.
Reduces Engine Wear
Vehicle thermostat reduces engine damage because its support engine reaches its proper operating temperature quickly. This limits fuel condensation on cold cylinder walls and reduces oil dilution.
If you have a properly functioning thermostat in your vehicle, it will also prevent moisture and sludge from building up in the crankcase. Overall, engine internal parts can work with the correct oil flow and component clearances to stay in good condition.
How Thermostat Works in a Car?
1. Engine Starts Cold
If you start a cold engine, the coolant remains below the thermostat’s rated opening temperature. The water pump will start to circulate coolant through the engine’s cooling passages. But thermostat will keep the main radiator circuit closed. This allows most of the coolant to flow to the radiator, so the engine can warm up faster.
2. Thermostat Remains Closed
A thermostat contains a small cylinder filled with wax which reacts to engine heat. As the engine is cold, the encapsulated wax inside the thermostat will stay in solid form unless it reaches its expansion point. Therefore, thermostat also remains closed at this stage.
3. Coolant Flows Through Bypass
The main thermostat valve is closed and blocks the coolant flow toward the radiator. Hence, the engine liquid will travel through the internal passages and small bypass channels back to the water pump. This lets the coolant circulate within the engine and related cooling passages.
During this stage, the coolant does not flow through the radiator. Hence, it does not release heat to the outside air. The bypass circuit spreads heat through the engine, so it warms up to its operating temperature much faster.
4. Thermostat Starts Opening
When the temperature of the engine starts to increase, the coolant will get hotter and reach the thermostat’s opening temperature. This will melt the wax inside a small brass or copper cylinder. The cylinder (also called a wax pellet or motor) contains a rubber insert and a piston. The wax expands and pushes the attached piston (rod) outward.
This movement will open the thermostat valve and allow coolant to flow through the cooling system to the radiator. Many thermostats start opening at around 82°C (180°F). At this point, it allows some hot coolant from the shorter circuit to mix with cooler coolant from the longer circuit. The purpose of this mixture is to keep the engine near its optimal operating temperature.
5. Finally It Fully Opens
In case, the mixture of hot coolant from the engine and cooler coolant from the radiator does not provide enough cooling. Then, the temperature will continue to rise and thermostat will open further. If the coolant reaches around 90°C (195°F), the thermostat fully opens the longer circuit (radiator circuit) and closes the shorter circuit (bypass circuit).
Sometimes, thermostat reaches its full open position at 95°C (200°F). The coolant then flows mainly through the radiator with the help of coolant hoses, where it releases maximum heat before it returns to the engine. Modern automotive thermostats use this same basic operating principle. However, engineers can adjust the thermostat so it begins opening at the required temperature for a particular engine.
6. Thermostat Closes As Engine Cools
Finally, the coolant temperature will drop below the thermostat’s closing range. A car thermostat mostly starts closing if the coolant temperature falls around 3 to 5°F below its rated opening temperature. For example, a 180°F thermostat can start closing at around 175°F (79°C). The wax has a high coefficient of thermal expansion, which means it now contracts as temperature decreases.
As a result, the return spring will move the valve back toward its closed position. This reduces coolant flow through the radiator and restores more flow through the bypass circuit. Thermostat does not simply switch between two fixed positions. It continuously adjusts its position based on coolant temperature rises and falls.
Where is Thermostat Located in Car?
Thermostat is not present in the same place on every vehicle. Its exact location depends on your vehicle make, model, engine design, coolant flow layout and water pump location. In most cases, you can find it where a coolant passage from the engine connects to the radiator circuit.
Near Water Pump: Some engines have thermostats close to the water pump. This setup mostly appears on engines where the water pump connects to the timing belt system.
At Engine Coolant Outlet: In many vehicles, you will find the thermostat inside a small metal or plastic housing. This housing preset where the radiator hose (mostly upper) connects to the engine. Basically, thermostat hoses connect the engine’s thermostat housing to the radiator.
Inside Thermostat Housing: You should look for a bolted housing attached to the engine. Thermostat generally located inside this housing, so you can not see the thermostat itself until you remove the housing.
Check Service Manual: If you still cannot identify the location, we recommend that you use your vehicle’s owner or service manual. It can show the exact position with a diagram for your specific engine.
Symptoms of a Faulty Car Thermostat
Over time, thermostat and its related parts can wear out or fail. The information in this table will help you to find out common signs of a defective thermostat and its possible reasons.
| Bad Thermostat Symptoms | Causes |
| Engine overheating | Thermostat stuck closed, valve binding or broken return spring |
| Slow warm-up or overcooling | Thermostat stuck open, weak internal spring & failed rubber seal |
| Erratic temperature gauge | Sticky valve mechanism, thermal spring fatigue, intermittent mechanical binding |
| Cabin heater problems | Thermostat open or premature coolant circulation |
| Coolant leaks around thermostat housing | Blown housing gasket, high system pressure, cracked housing or corroded seal |
| Check engine light activated (Code P0128) | Thermostat stuck open, slow response time & failed thermal wax element |
| Strange sounds, rumblings, knocking or steam from hood | Thermostat stuck closed, severe coolant boiling & extreme thermal expansion |
Can You Drive with a Bad Thermostat?
If you can drive with a faulty thermostat or not depends on how it fails. For example: if thermostat gets stuck closed, it can make the engine overheat quickly. This will cause serious engine damage, so it can be dangerous for you to drive.
In case the thermostat gets stuck open, you can drive for a short distance. But the engine can run below its proper operating temperature, which can reduce fuel efficiency. Hence, we advise you to only drive in an emergency to reach a mechanic, but do not take long trips.
Does a Bad Thermostat Affect Gas Mileage?
A malfunctioning thermostat can directly reduce gas mileage. Because it will cause the engine to run either too cold or too hot. A stuck-open thermostat forces the engine computer to pump excess fuel.
On the other hand, a stuck closed thermostat disrupts combustion and increases internal friction. Ultimately, both failure scenarios can cause the vehicle to burn more fuel than normal.
Frequently Asked Questions
What Are Single Valve and Double Valve Thermostats?
Single valve and double valve (dual valve) thermostats are important parts of a vehicle’s cooling system. A single valve thermostat uses one valve controlled by a wax-filled capsule. But the double valve thermostat has one wax capsule to control two valves at the same time.
Can You Drive Without a Thermostat?
It is possible to drive without a thermostat. But it is highly discouraged by professionals. Because it can increase engine wear, reduce fuel economy, increase emissions and provide less cabin heat. In some cases, removal of thermostat can also disturb coolant flow.
When Should I Change Thermostat in My Car?
You only need to replace your vehicle’s thermostat if it fails or shows signs of a problem. Unlike some maintenance parts, a thermostat does not have a fixed mileage or time for replacement.
Wrapping Up
So you can see how thermostats maintain the right balance in your car cooling system as driving conditions change. If this small device works correctly, the engine can operate within its normal thermal range without unnecessary stress.
As explained above, thermostat’s required opening temperature can vary slightly. In most cases, it is marked directly on the thermostat. Some modern vehicles use electronically controlled thermostats. They use a more complex control system than a traditional wax-operated thermostat.
