The thermostat in your engine has little to do with your engine’s running temperature. This misconception often confuses DIYers and hot rodders.
The issue comes from how thermostats—also called “T-stats”—are sold in different temperature ratings. This is the temperature that dictates the opening and closing of the valve. The engine’s running temperature is a function of the entire cooling system and engine parameters. The thermostat only maintains a minimum running temperature.
There are two main types of thermostats used in today’s engines:
Mechanical:
Most engines use a mechanical thermostat, where a wax pellet expands after the coolant reaches a certain temperature, thereby allowing the valve to open.
Electro-mechanical (EM):
EM thermostats, which have become more common since the early 2000s, also use a wax pellet. But the pellet temperature is controlled by the engine’s computer. This gives the ECM better control over engine temperature.
Regardless of type, all thermostats have the same function: to allow coolant to flow through the cooling system, maintaining a minimum temperature. When an engine is cold, it is inefficient. The fuel is not fully burned, the oil is too thick, and the engine itself is too cold to operate correctly.
The engine needs to warm up quickly for efficiency, emissions control, and to evaporate water (in the oil). Below about 160 degrees, the ECM is in open-loop mode. The oxygen sensors are not warmed up, and the ECM sends more fuel to the engine to bring up the engine temperature. Modern engines are the most efficient between 200 and 220 degrees Fahrenheit.
Typical Thermostat Problems
A typical mechanical thermostat has a wax pellet inside the center tube that expands and contracts, opening and closing the valve based on temperature.
When a thermostat fails, it’s usually a slow failure. Over time, the wax loses its ability to expand and contract. Eventually, the thermostat gets stuck. A T-stat that is stuck closed causes your engine to overheat.
A T-stat that is stuck open takes much longer to reach operating temperature. This is normally caught during the winter months, as you need to use the heater. You are less likely to notice long warm-up times during the warmer months.
If the thermostat is stuck closed, the engine warms up and then keeps getting hot until it overheats, and the engine shuts down. This can cause a blown head gasket or a complete engine failure.
A partially stuck thermostat causes some of both problems. The engine may heat up quickly and then overheat due to limited coolant flow. Or it can take a long time to heat up, and then slowly overheat. Either way, the cooling system is not operating correctly.
How Long Do Engine Thermostats Last?
Most car manufacturers suggest changing the thermostat every five years or 60,000 miles.
Thermostats have a life of about 10 years. However, the chance of failure increases dramatically after five years. Most car manufacturers suggest changing the T-stat every five years or 60,000 miles. Considering the price of a typical thermostat is less than $20, this is cheap insurance for your engine.
Electronically controlled thermostats are more expensive. They range from $25 to $100 based on the make and model.
How long does it take to change an engine thermostat?
For most vehicles, this project takes less than 30 minutes. If you are flushing the coolant, expect to spend about an hour to complete the full job. Most thermostats are easily accessible and do not require removing other components.
Why do thermostats have temperature ratings?
Manufacturers design engines to operate at certain temperatures, but the optimum temperature varies by each engine. Thermostats have a standard size, but the temperature ratings vary. In most cases, you should use the same temperature T-stat that your vehicle came with.
There is no reason to use a colder thermostat than the original temp. The only result is the coolant flowing slightly sooner. The engine still runs at the same temperature once the T-stat has opened. The only time a colder thermostat is needed is for race cars with overhauled cooling systems.
Finding the Right Thermostat for Your Vehicle
Most engines use a thermostat installed into the water neck. You should replace the gasket each time you remove the thermostat.
You can’t change between mechanical and EM T-stats. They must match the OEM spec. This makes selecting the right thermostat easy. Go to the
eBay Parts Finder
- opens in new window or tab.
, enter your vehicle details, and eBay will provide the thermostat listings that fit your vehicle.
The best brands of thermostat include AC Delco, Gates, Mahle, Motorad, SKP, and Stant. If you buy a name-brand thermostat, you can be sure that a 190-degree thermostat opens at 190 degrees—not 10 degrees too cold or too hot. Thermostats are inexpensive. It’s easy to choose a good one.
Should you bundle other work when you’re changing your thermostat?
Yes. Consider servicing your entire cooling system at the same time. Most components in the cooling system have similar life spans. The thermostat, hoses, and coolants should be changed every five years or 60,000 miles.
The tasks include checking and replacing your radiator hoses, heater hoses, and thermostat, as well as performing a coolant flush and change. OAT coolants (yellow, purple, pink, orange, and blue) should be changed every three to five years. IAT coolants (green) should be changed every two years.
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This article is meant to provide general guidance only. Automotive maintenance, repair, upgrade, and installation may depend on vehicle-specifics such as make and model. Always consult your owner's manual, repair guide for specific information for your particular vehicle and consider a licensed auto-care professional's help as well, particularly for advance repairs.
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