Coolant Thermostat

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By Reinier (Contact Me)
Last Updated 2022-10-25
Automobile Repair Shop Owner

What Does the Coolant Thermostat Do?

Both conventional and mapped thermostats are thermal valves that regulate the flow of coolant through an engine as a means of maintaining an engine’s temperature within a pre-defined range of acceptable temperatures.

Why is the Coolant Thermostat Needed?

Modern engines are designed to operate most efficiently at a temperature that is commonly known as their “operating temperature”, which is a nebulous and confusing concept. The problem with this concept is that it implies that all engines have an optimal working temperature at which combustion is “almost” complete, which in turn, implies that all engines are most fuel-efficient at their optimal operating temperatures- which is not always true of all engines under all possible operating conditions.

However, the fact is that although internal combustion engines are indeed more fuel-efficient when they are hot than when they are cold, any given engine’s optimal temperature has as much to do with limiting that engine’s oil consumption than with improving the combustion of fuel in the cylinders. This is because when an engine is cold, the pistons and piston rings get smaller as they contract “inwards”, while the cylinder bores get slightly bigger as they contract “outwards” as the engine cools down.

So, when a cold engine starts, the dynamic clearances between the pistons, piston rings, and cylinder walls are relatively large, which translates into poor cylinder sealing that increases oil consumption at low engine temperatures. As the engine warms up, however, the pistons, piston rings, and cylinder bores all expand towards each other, which reduces the dynamic clearances, thereby improving the seal between the piston rings and the cylinder walls, which translates into reduced oil consumption.

The above is true of all internal combustion engines, but some engines take much longer to warm up than others, meaning that some engines burn oil for longer periods than others do. Nonetheless, to reduce engine warm-up times, all engines are fitted with thermostats that restrict or close off the coolant circulation pattern while the engine warms up. This allows the engine to warm up in less time than it would have done had the coolant circulation pattern not been restricted or closed off by the thermostat.

In practice, all thermostats are calibrated to open when the engine coolant reaches a certain temperature. So when the thermostat opens in response to the temperature of the coolant, the coolant pump forces the coolant to circulate through the engine to absorb the engine’s heat and then shed the accumulated heat to the atmosphere when it passes through the radiator.

As a practical matter, the heating/cooling cycle [of the engine coolant] regulates the engine’s temperature, albeit with large fluctuations in the range of acceptable engine temperatures caused by the normal operation of the thermostat, which opens and closes by turns. Note that while this process generally prevents the engine from overheating, the normal operation of some conventional thermostats creates engine temperature fluctuations that can be as big as 80 to 100 degrees (F).

Although large fluctuations in engine temperatures in themselves are normally not harmful to engines, a large reduction in an engine’s temperature has a measurable negative effect on how fuel combusts in the cylinders. In practice, this means that on engines with conventional thermostats, the engine management system cannot “leverage” consistently high engine temperatures to maintain efficient combustion as a means of saving fuel and reducing emissions over extended periods. This is because the thermostat will open to increase the flow of the engine coolant when the engine reaches a predefined threshold to cool the engine down.

Until comparatively recently, this was an acceptable trade-off between preventing engines from overheating and (some) engines possibly not meeting emissions regulations during revised test-drive cycles. However, the recently revised Euro 6 and proposed Euro 7 emissions regulations have removed this trade-off. From an engine management perspective, however, the upside is that new types of thermostats now make it possible to control an engine’s temperature not only more precisely than is possible to do with conventional thermostats, but also independently of the coolant temperature- all without running the risk of the engine overheating. See the following section for details of how this works.

How Does the Coolant Thermostat Work?

Conventional thermostats have changed very little, if at all since they were first invented many decades ago. Conventional thermostats still rely on either a bi-metal strip* that acts as a reversible spring or a wax pellet that expands and contracts to open and close a valve plate that controls the flow rate of the engine coolant through the engine as a means of regulating the engine’s temperature.

* Thermostats that use bi-metal strips are now becoming exceedingly rare, but they are still in use as original equipment in some classic cars.

In all cases, conventional thermostats are fitted to an engine in such a way that the part of the thermostat that contains the wax pellet is in direct contact with the engine coolant. As the coolant warms up, some of the coolant’s heat is transferred into the solid wax pellet, which then melts and expands (to several times its volume in its solid state) at a rate that is roughly proportional to the rate at which the coolant’s temperature rises.

Although a strong progressive spring holds a conventional thermostat in the closed position, the force exerted by the melting wax eventually becomes high enough to overcome the spring tension, which forces the valve plate off of its seat, thereby opening the valve and allowing the coolant to start circulating through the engine under the force of the rotating coolant pump.

The converse happens when the coolant sheds heat to the atmosphere. The now-cooler coolant acts on the wax pellet, which partly solidifies again in the presence of the colder coolant. This forces the valve plate to close partially under the tension of the spring, thereby restricting the flow of the coolant through the engine.

As mentioned elsewhere, all conventional thermostats are calibrated to open and close at different temperatures, but this function is not controlled or determined by the strength of the spring. This function is determined by the composition of the wax pellet, so by simply creating different wax formulations, engineers can design thermostats that open and close at any point(s) between about 160 degrees (F) and about 210 degrees (F), which is the range of optimal operating temperatures for most modern car engines, which brings us to-

Mapped thermostats

Although mapped (electronically controlled) thermostats also use expanding wax pellets that are immersed in the coolant to force the valve plate open against spring tension, mapped thermostats do not rely on the temperature of the coolant to work. Instead, mapped thermostats use built-in heating elements that are controlled by the PCM, to supply heat to the wax pellet.

The other major difference between conventional and mapped thermostats involves the calibration of mapped thermostats. As a practical matter, mapped thermostats do not only open later than conventional thermostats to maintain higher engine temperatures, but they are also calibrated to work in two distinct temperature ranges that are based on the engine load. Here is how this works-

At low to moderate engine loads, a mapped thermostat will only open when the coolant temperature is between 212 degrees (F), and 230 degrees (F), depending on both the vehicle and the thermostat’s nominal operating range. The higher engine temperature allows for vastly improved combustion of the fuel, thus reducing harmful exhaust emissions.

At high engine loads, or when the “Sport” or “Performance” mode is activated on some vehicles, the PCM will activate the thermostat’s heating element earlier to open the thermostat at temperatures below 212 degrees (F). Depending on both the vehicle and the thermostat’s operating range, this could typically occur when the coolant temperature is between about 185 degrees (F), and about 203 degrees (F). The reduced temperature increases the density of the intake air, which improves performance and prevents the engine from overheating, at the same time. The PCM could also activate the radiator cooling fan(s) at this time to help in reducing the engine’s temperature further.

Regardless of the engine load, though, the PCM will monitor the coolant temperature continuously, which information it will use to supply the mapped thermostat with pulse width modulated signals to maintain the engine’s temperature in a much narrower range than is possible to do with a conventional thermostat.

Note though that many late-model vehicles are also fitted with an electric coolant pump that, when used in conjunction with a mapped thermostat, makes it possible for a PCM to control an engine’s temperature in ranges that vary by fewer than five degrees.

Where is the Coolant Thermostat Located on the Engine?

This image shows the location (arrowed) of the thermostat on a 2008 V6 Toyota Camry engine. Note though that while it might appear as if the thermostat in this example is easy to access, bear in mind that this picture shows the engine removed from the vehicle, and partially disassembled.

In practice, it can sometimes be challenging to access the thermostat and its housing on some engines, since it may be necessary to remove or partially disassemble unrelated engine components such as the radiator, radiator hoses, radiator fans, and A/C lines to gain access to the thermostat. Note also that although conventional thermostats might appear to be similar in appearance, thermostats often differ in ways that may not be immediately apparent to non-professional mechanics. This includes the fact that many thermostats are calibrated to be vehicle make and model specific, meaning that for the most part, thermostats are NOT interchangeable even if a given thermostat fits on multiple engines.

If you are uncomfortable with the idea of removing major engine components or are afraid of installing an unsuitable thermostat in your car’s engine, the wisest course of action would be to seek professional assistance with diagnosing and replacing a suspect thermostat, due to the very real possibility that a mistake could cause the engine to overheat fatally.

What Does the Coolant Thermostat Look Like?

This image shows an example of a typical conventional thermostat, but it should be noted that even though thermostats are usually mounted on the cylinder head on almost all engines, the thermostat is enclosed in a metal or plastic housing that connects to the top radiator hose, and the thermostat is therefore not visible unless the housing is removed from the engine.

The image below shows an example of a mapped thermostat, but note that unlike conventional thermostats and their housing all follow a general pattern, the appearance of mapped thermostats varies greatly between vehicle makes and models-

For example, the mapped thermostat shown here is not representative of the appearance of mapped thermostats in terms of both design specifics and materials used in their construction. As a matter of convenience, the quickest way to locate mapped thermostats on some engines is to look for an electrical connector on a plastic or aluminum object that connects to a radiator hose.

Note that while some mapped thermostats may connect to multiple radiator hoses, the one design feature all mapped thermostats have in common is the presence of an electrical connector. Thus, if you find an electrical connector on an object that connects to one or more radiator hoses, you’ve found the mapped thermostat.

What are the Symptoms that the Coolant Thermostat is Bad?

The most common symptoms of defective or failing thermostats are largely similar, if not always identical across all vehicle makes and models. Moreover, while some symptoms of thermostat failures can be dramatic, and very expensive to repair, not all internal combustion engines always fail catastrophically when they overheat, but be aware that all internal combustion engines suffer at least some damage when they do overheat seriously.

Note also that the type of damage any given engine could potentially suffer as the result of a thermostat failure or malfunction depends on both the engine and the nature of the thermostat failure. Below are some details of the possible symptoms of failed, defective, or malfunctioning thermostats, but let us start with possible symptoms that could occur when-

The thermostat is stuck open

  • If the engine is fitted with a mapped thermostat, this condition will set one or more dedicated fault codes, and possibly also cause a warning light to illuminate
  • The engine may never reach its optimal operating temperature because the fast-moving coolant sheds the engine’s heat too quickly for the engine to warm up
  • The engine could overheat fatally because the fast-moving coolant does not spend enough time in the radiator to shed its heat to the atmosphere
  • Fuel consumption could increase dramatically because if the engine does not reach its optimal operating temperature, the engine management system may continue to enrich the air/fuel mixture to improve combustion, especially if the coolant temperature remains below about 100 degrees (F). Note that recurring catalytic converter failures could also occur if this condition is allowed to persist over extended periods
  • Oil consumption could increase noticeably or even significantly if the engine does not reach its optimal operating temperature because dynamic operating clearances remain excessively large

The above symptoms do not typically progress into engines failure if the problem is corrected promptly, but more serious consequences usually follow when-

The thermostat is stuck closed

  • Stored trouble codes and one or more illuminated warning lights will be present
  • Depending on the vehicle, the engine may overheat only under some operating conditions, such as when driving in slow-moving traffic, or when towing a trailer
  • In many cases, such as when the thermostat fails unexpectedly, the engine will typically overheat fatally in only a few seconds
  • Excessive pressure in the cooling system caused by engine overheating could cause radiator hoses and/or heater hoses to burst or rupture, which usually leads to catastrophic engine failure
  • In cases where the engine does not fail catastrophically as a result of overheating, typical engine damage includes blown cylinder head gaskets, damaged pistons, and piston rings, and damaged coolant pump and radiator
  • Serious engine overheating always causes engine oil to degrade to the point of uselessness
  • Both drive belts and timing belts often harden and crack/split after an episode of serious engine overheating

How do you test the Coolant Thermostat?

In the case of mapped thermostats, it is possible to test the electrical integrity of the unit by using a scan tool with bi-directional control functions to simulate the control inputs a PCM might make. Note though, that even if such a thermostat passes this test, there is no guarantee that the thermostat will work as expected when it is exposed to the heat and pressure of an engine cooling system.

Similarly, testing a conventional thermostat by placing it in a pot of hot or boiling water to see if it opens (or not) does not produce unambiguous proof that the thermostat will work when it is exposed to the heat and pressure of an engine cooling system.

Both test methods described above might produce positive results, but it is worth remembering that when a thermostat shows signs of failing when it is installed, testing it in any way outside of the engine is pointless, because it will almost certainly fail completely soon, if not immediately after it is reinstalled.

The most effective way of avoiding thermostat failures and the huge repair bills that come with such failures is to replace the thermostat every two years, or more regularly in hot climates, and then only with OEM or OEM-equivalent parts sourced from reputable and trusted parts outlets.

Regardless of their design, thermostats are and have always been primitive devices that operate in extremely harsh environments. Thus, expecting a thermostat in any engine to operate as expected for many years, or for the life of the vehicle (whichever comes second), is not only unrealistic- it is also akin to inviting engine failure especially if the composition and condition of the coolant are not maintained.

How do you replace the Coolant Thermostat?

While it may be possible for most non-professional mechanics with some experience in performing critical engine repairs to replace a thermostat on almost any light vehicle engine that is older than about 20 years, it is not a procedure that we would recommend on post-2010/11 engines.

Given that much, if not always all the coolant is often lost during the thermostat replacement process, it should be noted that refilling the cooling system on most late-model vehicles is not as simple as pouring a mixture of water and anti-freeze into the expansion tank.

In many cases, the cooling system can only be refilled successfully with specialized equipment that draws a vacuum in the system to prevent air bubbles from forming and becoming trapped in the coolant circulation pattern. Moreover, on many engines, it is necessary to purge air from the cooling system by following OEM-prescribed procedures; failing to perform these steps, or failing to perform them in the correct order is almost guaranteed to produce engine overheating, and very often, subsequent engine failure.