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When Is it Time To Replace My Engine Thermostat?

Automakers don't specify a mileage when thermostats should be replaced. We discuss how they operate, and cover warning signs that it's time to replace your thermostat.

When you consider that temperatures within an engine's combustion chambers reach over 2,000 degrees Fahrenheit, it's a marvel to consider how effective engine cooling systems are at removing that heat - keeping things within a "mild" 180-200 degree range.

A typical cooling system includes a water pump which circulates liquid coolant, channels within the engine block for the coolant to flow through and pick up combustion heat, a radiator that disburses coolant heat to the outside air, a heater core that disburses heat to the passenger compartment, and a thermostat (mounted in a housing on the engine block) which serves as a switching station to direct coolant to different locations as needed.

Typical Thermostat Location On Engine
Shown here is a typical thermostat location on an engine.

The liquid coolant itself is typically a 50/50 mixture of distilled water and chemicals such as ethylene glycol. Because this mixture serves to lower the freezing point of pure water, it's also described as "anti-freeze". The chemical makeup of the liquid coolant, as well as the pressure it's under, also raise the boiling point to a higher temperature than pure water - allowing the system to operate at necessary temperatures without catastrophe. For more details on liquid coolant and the variations of it that exist, see our related article Antifreeze Explained: What It Does and Why It's Needed.

In this article, we'll focus on what thermostats do, how to tell when one is malfunctioning, and provide some guidance as to when they should be replaced. Vehicle manufacturers don't specify a mileage or date interval for thermostat replacement the way they do for an air filter or timing belt, because thermostats are not considered a routine maintenance item. Instead, they should be replaced "if and when the part malfunctions". Rather than wait for that to happen, we'll also advise regarding other good opportunities for replacement.

How A Thermostat Works

When we mentioned a thermostat serves as a switching station, we mean the thermostat is essentially a valve that opens and closes in proportion to the temperature of the coolant. When the engine is started cold, it's important for the sake of engine efficiency and emissions to get things warmed up as fast as possible. To do this, the thermostat will start off in the fully closed position - blocking any coolant flow into the radiator where it would lose heat. Instead, the coolant is redirected back into the engine block to ensure it reaches its ideal temperature (around 190 degrees) sooner - regardless of how hot or cold the outside air is.

Components Of Typical Engine Thermostat
The components of a typical engine thermostat.

The basic components of a thermostat are metal springs, flange, frame assembly, and a valve containing a piston embedded in a special wax. When the coolant starts warming up, the thermostat automatically begins to open. This is thanks to pressure from a piston that's pushed upward in the valve as a wax pellet melts and expands at a certain temperature, and from one of the two springs. Both of these mechanical methods are simple and durable, with little to deteriorate or break with age.

Typical Engine Thermostat Closed / Open
Here, a typical engine thermostat is shown in the closed position (left) and open position (right).

As the thermostat opens, the flow of coolant is gradually diverted into the radiator instead. Thermostats have a temperature "rating" (such as 180 or 190 degrees Fahrenheit) which is the temperature at which the thermostat begins to open. It will reach the fully open point approximately 15 to 20 degrees higher. This temperature rating will be stamped visibly on the thermostat somewhere. To prevent air in the cooling system from being trapped and creating hot spots, most thermostats have some kind of check valve that allows air to pass through and be released.

Temperature Rating Stamping On Thermostat
Thermostats usually feature a stamping that shows their temperature rating.

Some Thermostats Are Integrated Into Housing Assemblies

Since the turn of the millennium or so, many automakers have transitioned to thermostats that are part of an integrated housing assembly located on the engine block or cylinder head. A pipe on the housing acts as the coolant outlet. Traditional stand-alone thermostats also sit inside of housing assemblies - however, they are replaced independently of the housing.

Thermostats Permanently Integrated Into Housing Assembly
Today's vehicles often feature thermostats that are permanently integrated into a housing assembly.

An integrated thermostat works the same way as a stand-alone unit, with a benefit being that the integrated housing is easier to access and replace as a whole. This is a plus, because housing assemblies can also become damaged if a prior overheating condition occurred. Additionally, the likelihood of a thermostat being installed in the wrong position is eliminated.

Thermostat Select Vehicle Box

If you're not sure which type of thermostat your vehicle has, just enter the year, make, and model in our SELECT VEHICLE box along the top of the screen and hit the "Go" button. Our website will automatically narrow your search down to products specifically designed for your car or truck.

Signs A Thermostat Is Malfunctioning

Nothing lasts forever - and neither do thermostats. When they malfunction, they most often get stuck in either the open or closed position.

Failed Thermostat Blue Coolant Light
Some vehicles feature a blue coolant light that signifies the coolant has not yet reached full operating temperature. A thermostat that has failed "open" will result in coolant that never gets hot enough.

A thermostat that's stuck open will circulate coolant through the radiator at all times, even during cold starts. As a result, the engine may never reach full normal operating temperature, and the coolant temperature gauge may hover at a point that's colder than usual. This has a negative effect on combustion, which leads to unburned fuel contaminating exhaust and engine oil. You may notice the interior of your vehicle never gets very warm during winter months either, because the coolant running through the heater core isn't warm enough to produce heat. A thermostat fails "open" this way if the spring breaks or gunk in the system prevents the valve from fully closing.

Closed Failed Thermostat Sign
A thermostat that fails "closed" will cause an overheating condition.

Conversely, a thermostat that sticks in the closed position will completely block coolant flow to the radiator where heat is disbursed away to the outside air. Engine temperatures climb rapidly and dangerous overheating soon results. Typically, a thermostat fails "closed" this way if the wax pellet has been corrupted by corrosion or by a previous overheating condition.

Note that if your temperature gauge reaches the red line for any reason, the thermostat should be examined for any possible damage and considered for replacement. This is because metal housings may have warped and lost their original shape and the properties of the wax cylinder have been compromised. As you might imagine, a thermostat suffering this kind of damage won't be able to function normally and do its job.

Sometimes, a malfunctioning thermostat will stick in a partial position, or it may open and close erratically in spurts. Coolant temperature will fluctuate, and you may see a gauge reading that changes north and south of the center point where it should be. Depending on your make and model, improper temperature gauge readings can trigger a check engine warning light as well.

Opportune Times To Replace A Thermostat

As we mentioned before, automakers shy away from prescribing a set time or mileage to replace a thermostat. We tend to agree that thermostats normally last a long time, and so replacing them too soon might not be the best use of your hard-earned car parts money.

Thermostat Replacement Process

However, there are multiple reasons why you should consider replacing the thermostat BEFORE it fails:

  • The part is relatively inexpensive (many on our site are under $25)
  • On many engines, the part is easily accessible
  • Waiting until it fails can multiply the repair cost exponentially (an overheated engine might need internal repairs costing thousands of dollars)
  • The thermostat can, and should, be replaced when performing other cooling system service work, such as replacing the water pump or radiator hoses, or flushing the coolant

There are times when the job of replacing a thermostat is easier and more logical as a preventative measure. For example, if you're draining and replacing the coolant in your radiator, or you're having a new water pump installed, replacing the thermostat during this time makes sense because the thermostat is in the same area, and, you need to drain the coolant anyway. For many vehicles on the road today, automakers have switched over to 100,000-mile coolant, so, a new thermostat at the same time is a simple, inexpensive, and wise investment.

Thermostats We Offer

For starters, we've got individual thermostats for a wide range of vehicles including the AC Delco Professional Engine Coolant Thermostat, Gates Engine Coolant OE Thermostat, and Stant Engine Coolant Thermostat just to name a few.

For GM vehicles back to 1987, there's the AC Delco GM Original Equipment Engine Coolant Thermostat & Housing Assembly. Chrysler brand owners (including Jeeps) can check out the Crown Thermostat and Mopar Engine Coolant Thermostat. For Hyundai & Kia models back to 1996, the Auto 7 Engine Coolant Thermostat has you covered. The Beck Arnley Thermostat is made for a wide range of European and Asian vehicles back to the 1950s, and the Genuine Engine Coolant Thermostat is designed for select Porsche, Jaguar, and other European makes. If you've got a Jeep model dating back to the 1940s, take a look at the Omix-ADA Thermostat.

If your vehicle is equipped with a thermostat that's built into the housing, the Gates Engine Thermostat with Housing is offered for a wide variety of vehicles, along with the Four Seasons Engine Coolant Thermostat & Housing Assembly. Beck Arnley's Coolant Thermostat & Housing Assembly and Genuine Engine Coolant Thermostat & Housing Assembly specialize in import vehicles. The Motorcraft Coolant Thermostat Assembly focuses on OEM equipment for Ford division models.

Replacement Thermostats Variety Gallery

When you consider how inexpensive thermostats are compared to an entire engine, installing a new one before a problem starts is a smart-money purchase that will pay you back with peace of mind and many trouble-free miles. We're here to help with any questions you may have on the thermostats we sell, so give us a call seven days a week!

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