The Engine Thermostat: What Is It, What Does It Do, And When To Replace It

The Engine Thermostat: What Is It, What Does It Do, And When To Replace It

Despite its small size and unassuming appearance, the thermostat plays a crucial role in regulating the engine's temperature and preventing overheating.

In this post, I'll demystify this often-overlooked component and explain how it works and what happens when it fails and when it should be replaced.

Regulating Engine Temperature

The engine thermostat is a valve that is located between the engine and the radiator.

It's generally located within a metal or plastic housing that is bolted to the engine. The upper radiator hose then connects to this thermostat housing.

In a nutshell, its job is to regulate the flow of coolant from the engine and the radiator by opening and closing based on the temperature of the engine.

Here are the specifics of its operation:

  • Cold engine start: The thermostat is closed, which forces the coolant to circulate within the engine to warm it up. Coolant is not allowed to circulate into the radiator.
  • Engine begins to warm up: As the engine begins to warm up, the coolant in the engine block absorbs heat from the engine components and expands.
  • Engine at operating temperature: The thermostat opens, allowing the coolant to flow through the radiator and cool the engine down.
  • Coolant circulating in the radiator: As the coolant circulates in the radiator, it's cooled by the air passing over the radiator fins. Since the thermostat is still in its open position, the cooler coolant now flows from the radiator into the engine.
  • Engine temperature decreases: As the cooler coolant circulates in the engine, it absorbs heat from the engine and cools it down.
  • Thermostat response: As the temperature of the coolant continues to drop, the thermostat responds by closing and restricting the flow of coolant from the engine into the radiator.
  • Engine temperature increases: With the thermostat closing, the coolant pressure and temperature (in the engine) begin to increase. At 195 °F, the thermostat opens and allows coolant to circulate from the engine and into the radiator.
  • Wash, rinse, repeat: The thermostat will continuously cycle between opening and closing as the engine operates to maintain a consistent engine temperature, which helps to optimize fuel economy, reduce emissions, and ensure optimal engine performance.

How The Thermostat Does What It Does

The thermostat works based on a wax pellet that is located inside the valve. The pellet is made of a material that is sensitive to heat and expands when it's heated by the engine coolant.

The expansion of the wax pellet causes the valve to open, allowing the coolant to flow through from the radiator.

When the engine cools down, the wax pellet contracts, causing the valve to close and preventing the coolant from flowing through the engine.

Engine Thermostat Temperature Ranges

Engine thermostats are available in different temperature ranges. The most common temperature ranges are:

  • 195°F
  • 180°F
  • 160°F

The temperature range of the engine thermostat you need to buy will depend on the specific make and model of your vehicle.

The best way to find this information is to consult your vehicle's owner's manual, which should provide details on the recommended thermostat temperature range for your engine.

If you don't have access to the owner's manual or the information isn't provided, you can consult the parts catalog at your favorite automotive parts retailer to determine which thermostat to buy.

It's important to note that in many newer vehicles, there may not be an option for a lower temperature thermostat, so you'll need to stick with the OE recommended temperature thermostat that's available at your automotive parts retailer.

Does The Engine Need A Thermostat?

The proper operation of an engine is highly dependent on maintaining a specific temperature range. This temperature affects everything from the engine's efficiency and gas mileage to its emissions output.

Without a thermostat to regulate the flow of coolant, the engine may suffer from a variety of problems, including overheating, decreased performance, increased emissions, and in the long run, excessive wear and tear on internal engine components.

So although it's technically possible for the engine to run without a thermostat, it's not recommended.

Symptoms Of A Bad Thermostat

A faulty thermostat can cause a variety of problems, including engine overheating and poor performance. It's essential to recognize the symptoms of a bad thermostat to prevent serious engine damage.

If the thermostat is stuck closed, it can cause the engine to overheat, which can lead to engine damage.

If the thermostat is stuck open or missing, you'll see one or more of the following:

  • Richer air/fuel mixture: When a thermostat remains stuck in an open position, the engine runs cooler than intended, which prompts the powertrain control module (PCM) to inject more fuel into the engine, resulting in a richer air/fuel mixture.
  • Check engine light illuminated: In some vehicles, the cooler engine temperature due to a stuck open or missing thermostat will cause the PCM to set a diagnostic trouble code and illuminate the check engine light.
  • Longer engine warm-up time: A stuck open or missing thermostat prolongs the engine's warm-up time, which reduces fuel efficiency and causes more wear on engine components.
  • Increased emissions: A stuck open or missing thermostat causes a richer air/fuel mixture, which can lead to higher emissions of harmful pollutants, including hydrocarbons (HC), carbon monoxide (CO), and nitrogen oxides (NOx).
  • Poor fuel economy: When a thermostat remains stuck in an open position (or its missing), the engine runs cooler than intended, which forces the ECM to inject more fuel into the engine, causing poor fuel economy.
  • Inadequate cabin heating: The thermostat regulates the flow of coolant through the heater core, which is responsible for warming the vehicle's cabin. A stuck open or missing thermostat can prevent the heater from working properly, which is especially inconvenient during colder months.
  • Increased piston ring wear: A stuck open or missing thermostat causes a richer air/fuel mixture that can dilute the engine oil with gasoline. This can increase wear on engine components such as piston rings, leading to higher oil consumption and decreased engine performance.
  • Black exhaust smoke: When a thermostat remains stuck open, the engine can run excessively rich, leading to the emission of black smoke from the tailpipe. This can also cause damage to the vehicle's catalytic converter and oxygen sensors.

When To Replace The Engine Coolant Thermostat

The engine coolant thermostat has a finite lifespan that cannot be extended, and there are no tests available to determine if it has reached the end of its service life.

Even worse, vehicle manufacturers generally don't provide a recommended replacement interval to ensure the thermostat doesn't cause catastrophic overheating engine problems.

Most folks only replace the thermostat when it fails, which can be a costly mistake. A failed thermostat can cause a blown head gasket, resulting in extremely expensive repairs.

Avoiding costly repairs due to a failed thermostat can be achieved by replacing it preventively. This can be conveniently done during other maintenance procedures, such as:

  • When replacing the water pump.
  • When flushing the coolant.
  • When replacing the radiator hoses.

Engine Coolant Thermostat Diagnosis

Before testing the thermostat, it's important that other potential issues be ruled out, such as:

  • Coolant leaks.
  • Fan clutch or fan motor problems.
  • Blown head gasket issues.
  • Clogged radiator.

Once these issues are ruled out, the thermostat can be tested. The following tutorial, although for a mid-1990s vehicle, provides a basic testing framework, which can be adapted for different models and years:

Should I Do It Or Take It To Repair Shop?

If you are comfortable performing basic diagnostic tests and have the necessary tools, you can perform the diagnostic tests yourself.

However, if you are unsure or don't have the necessary tools, it's best to take your vehicle to a qualified mechanic or repair shop for diagnosis and repair.