Engine Coolant Thermostat


Its function is to control the flow of the coolant fluid between the engine and the radiator.



Used in the cooling systems of engines cooled by coolant liquid or air.


Operation Principle

The thermostat uses a thermo-expansive wax that is calibrated accordingly to a specification. When the temperature increases, the wax melts and expands inside the thermo-element, pushing the piston outward, compressing the spring and opening the valve, which allows the passage of the coolant to the radiator. (Fig.1).









Generally near the engine and the upper hose that exits from the radiator.



Used to:

  • Allowed fast heating of the engine (the thermostat remains closed while the engine is cool).
  • Keep the engine operating between the designed temperature limits. (after the thermostat opened).


  • Reduce wear and extend the engine’s life.
  • Reach the maximum torque and power.
  • Avoid excessive fuel consumption.
  • Reduce emissions increases.


Operating Temperature

Operating temperature depends on the car manufacturer’s specification, the opening temperature is usually found in the thermostat body.


The thermostat opening temperature will be, give or take 2 degrees of the nominal temperature and, should be completely open after 15 to 20 degrees over the nominal temperature, at this point the thermostat must be at least 8mm open. (Fig.2).







When it does not work:

  • Opened: High fuel consumption and emission, low power and torque.
  • Closed: Causes the engine to overheat, cylinder’s head gasket burn out, warping the cylinder’s head, etc.




Valve’s test: (Fig. 3).






  1. Place the thermostat into a container or pan with coolant liquid and leave it in the stove in medium temperature. (try do not allow the thermostat to touch the bottom of the container).
  2. Observe the thermostat operation, while using a thermometer and stirring the liquid for better temperature uniformity.
  3. After the liquid starts to boil, with for 5 minutes and, remove the thermostat and check if it is completely opened.

Note: the boiling temperature varies with altitude it will only boil at 100C at sea level

Important: This test verifies only the valve’s operation. A more detailed study, such as the assessment of the opening temperature, should be accomplished with specific equipment in the factory.



Important actions when changing the Thermostat:

  • Always use a new gasket or seal ring.
  • Bleed the air (remove air bubbles) from the cooling system.
  • Check for leaks after the repair.
  • Do not leave the vehicle without a thermostat, otherwise, the engine will always operate cold, causing failures, excessive fuel consumption, and increased emissions.



  • Always check the correct thermostat for each vehicle model.
  • Never perform maintenance repair while the cooling system is still hot. There is a high risk of burns.
  • At any evidence of excessive temperature, park the vehicle in a safe place and turn off the engine immediately, otherwise, you may damage the engine.
  • Check the coolant level weekly, with the cold engine.
  • Always use the specified coolant and the correct rate for your vehicle.
  • Do not fill the coolant reservoir only with plain water, because this will dilute the coolant concentration.
  • Any reduction in the coolant level may indicate a leaking in the cooling system.
  • Perform the preventive thermostats maintenance every 30.000 Km.



All MTE-THOMSON products are guaranteed for 01(one) year against defects from faulty materials or fabrication. The guarantee is limited to the replacement of the faulty part; we cannot extend the guarantee to cover defects caused by misuse, neglect, accident or wear and tear. We cannot accept liability for consequential loss or damage which is claimed to have resulted from the use of one of our products. More information:


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