Thermostats and How They Work in an Engine

The thermostat plays an absolutely vital role in the operation of your engine, yet it is not very well understood how it operates. With a clear understanding of the operation of the thermostat and how they work in an engine you will be better able to diagnose engine overheating and everything from engine cooling to heating the interior of your vehicle. These are all related to the thermostat in an engine!

When the engine is not running, when it’s sitting in your driveway doing nothing, the thermostat is in a closed position. Now, here’s where the confusion begins. To look at a thermostat right out of the box it appears to be in an open type of position. The spring is in its elongated position. This extended spring, or elongated spring, is the closed position when installed in your vehicle. The crude animation below reflects exactly what the thermostat is doing in your engine.

When the engine is started the thermostat is closed. As the fluid becomes hot the thermostat opens. When the engine temperature becomes cool again the thermostat closes. Normally a thermostat does not open and close as often as in this .gif. It is a much slower process. Once it is open for a drive, it will stay open or near to.

This opening and closing of the thermostat is to control the flow of coolant. When the engine is started only some of the coolant is allowed to flow. This allows for a faster warm-up. An engine doesn’t perform up to its optimum until the engine is fully warm. The thermostat is expediting the warm up process by restricting a portion of the coolant.

When the engine is warm the coolant allowed to flow is proportionate to the temperature of the thermostat. If the thermostat is rated to open at 165 degrees then that thermostat will open or close according to the coolant reaching 165 degrees. Some thermostats are rated higher and some lower. They are referred to by laymen as summer and winter thermostats.

If you were to remove the thermostat completely from your engine, the engine will still run fine. You will find that it takes a lot longer for the temperature gauge to reach operating temperature, thus the responsiveness and power under load may suffer a bit, but you wouldn’t notice unless you know your car very well. The concern is that over time the engine will suffer and have a reduced life from running when it is not warmed up for peak performance.


When you find your vehicle is overheating check the air conditioning settings. When you are in traffic, on a hot day, running the air conditioning, it is a heavy load especially on an older engine. Try turning off the air conditioning before the engine completely overheats and you may find that’s all it takes to bring the temp down to operating level.

If the air conditioning is off, a little trick you can try when your engine is starting to overheat is, turn the heater on. Yep, that’s right. Turn the dial to high heat and the fan to full blast. This will activate the heater core which is like a small radiator inside the vehicle cabin. It blows air across the heater core fins and can cool the coolant enough to keep the engine from overheating. Obviously, if it’s hot outside, you will have to open the windows wide to let the hot air out of the cabin. Unless you enjoy a good sweat.

Your best bet is to avoid overheating in the first place. This is done by maintaining a clean, well maintained, cooling system. To accomplish this adhere to the following list!

  • Flush the radiator periodically, especially if debris has entered the system.
  • Clean the front fins of the radiator by removing bugs from it. The fins must be bug free to allow air to flow through and cool the fluid inside.
  • Never put water into the radiator unless it is for the purpose of diluting an undiluted radiator fluid. Water will corrode and rust the inside of the radiator, water pump, thermostat and hoses before you can say, “Oh shit.” Water is the most destructive element you can put in your radiator!
  • Keep the radiator topped off. Check the coolant level once a week and top it off as needed.
  • Do not place anything in front of the radiator. It must have a clear path for air to flow through it.
  • Don’t remove the thermostat for any length of time.
  • Replace the components IE: thermostat, water pump, hoses, etc. as they go bad. The cooling system is in constant action when the motor is running. Parts will wear out. Replace them as soon as you have a sign of trouble.
  • Check the condition of the radiator cap periodically (every 6 months). The radiator cap must make an air tight seal on the radiator. If it does not water will evaporate and exit through the cap. This can reduce the coolant level to near nothing in a short drive. A damaged or non sealing radiator cap can cause problems like the heater not working, the engine suddenly over heating and a high loss of radiator fluid. An engine won’t necessarily melt if it over heats but if it over heats multiple times at a high temperature the metal in the engine will break down and the engine can warp. When it warps you will have a blown head gasket.

If you think your engine is about to overheat, pull over in a safe spot, turn off the engine and open the hood. Allow the engine to cool. Then add coolant to the system. Do NOT spray water on an overheating engine. Do NOT remove the radiator cap under any circumstances. Wait until the coolant hose lines are pliable (squeezable by hand) before opening. If the coolant hoses feel firm that means there is high pressure in them and if you remove the cap that high pressure will come out on you!

The Heater Core

The heater core we spoke about earlier, to reduce the heat in an overheating engine, this heater core is what warms the interior of your vehicle on cold days. It’s a miniature radiator installed at the passenger foot area behind and below the glove compartment. The warm radiator fluid flows through the heater core all the time. When you want to warm the cabin of your vehicle a fan blows across the heater core and the heat is blown into the cab and warms it up.

Heater cores, like radiators, go bad after time. Especially if you put WATER into your radiator. This habit will reduce the life of your heater core to a few years, if that. You will know your heater core is going out if:

  • The floor on the passenger side is wet.
  • If the wet floor smells like radiator fluid.
  • If the odor of radiator fluid is in the cabin of the car.
  • If your windshield fogs up and it’s the middle of the summer!
  • If the windshield becomes really foggy in the winter and it doesn’t clear up easily with the defroster.
  • The cabin environment becomes unusually muggy.
  • The radiator fluid needs topping off more often than usual.

Any one of these are signs your heater core is going out. Fortunately the heater core is not all that difficult to replace. Your biggest hurdle may be just accessing the unit. Some vehicles the heater core is tucked way up under the dashboard. On others it’s right there at the foot area of the passenger and easy to access. Here is our page on Replacing the Heater Core.

The Water Pump

The water pump does exactly what its name implies, it pumps. At one time, long ago, it did pump the water in the engine so its name was spot on. But now-a- days radiator fluid has replaced water. So a more appropriate name for the water pump would be radiator fluid pump.

The water pump has a pulley attached to the front of it. The pulley is activated by the serpentine belt. When the engine is running the serpentine belt causes the pulley to spin which in turn also causes the radiator fan to spin . The radiator fan is attached to the front of the pulley via a clutch.

There is no mistaking when the water pump goes out. The engine will overheat within minutes if not seconds. When the water pump is going out, but it is still pumping some fluid you can check the weep hole for drainage. In the most inconvenient, difficult location to view, on the front, underside, of the water pump there is a small hole. You will need a flashlight and a view between the fan blades and behind the pulley to inspect it. When the water pump starts to fail this hole will leak radiator fluid. It is usually a small amount of fluid but it will leave a definite streak from the weep hole down the water pump. When you see this streak of fluid, the pump is definitely on its way out. Here you can read up on How to Replace the Water Pump.

Locate your water pump with this Amazon link that will take you to the vehicle information page for a water pump search.

Water Pump Search

That is the cooling and thermostat operation in your vehicle in a nutshell.

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