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Basic information about thermostats

Thermostats regulate engine temperatures and keep them constant by controlling the flow rate of the coolant through the radiator, ensuring a balance between the heat introduced into the system and that released externally.

By accelerating the preheating of the engine and constantly regulating its running temperature, the thermostat prolongs the engine’s life, improves its performance and reduces fuel consumption and emissions.

It is normally housed in a plastic or aluminum manifold on the engine cylinder head.


Some thermostats are positioned in various parts of the cooling system to perform auxiliary functions:

- Cooling exhaust gases from the EGR system

- Regulating the temperature of the transmission fluid in automatic gearboxes

- Regulating engine oil temperature


Component parts and working principles

When the engine is cold started, the thermostat prevents the circulation of the coolant to the radiator, thus reducing the time it takes to reach the ideal temperature. Once the coolant temperature increases (to between 75°C and 125°C, depending on the type of engine and the position of the thermostat in the cooling system), the thermostat starts to open, allowing the fluid to pass through the radiator. This process occurs mechanically through the expansion of a mixture of wax and aluminium inside the sensing element in the thermostat, which opens a valve when it exceeds the activation temperature. The thermostat closes at temperatures of more than 5°C below the opening temperature to control the temperature range of the coolant inside the engine.


Main causes of thermostat failure:

- Failure to open due to corrosion of the components

- Insufficient coolant, caused by micro-leakage


Effects of thermostat failure

If the thermostat remains stuck in the rest position, the coolant will not flow to the radiator. When there is insufficient coolant, air bubbles form in the cooling system. Both situations can lead to burning of the cylinder head gasket.


Facet thermostat types

Facet offers a diversified range of thermostats, from the simplest to latest-generation map-controlled devices, to meet temperature control needs under various operating conditions and ensure perfect compatibility with different vehicle models.

Simple and bypass valve thermostats

These types of thermostat have a metal structure formed by a flange and the housing in which the sensing element, the valve and the tension spring are enclosed. The heart of the sensing element consists of a wax and aluminium mixture that expands under heat, creating a force that causes the sensing element to slide, thus opening the valve through which the coolant flows. The force exerted by the sensitive element is opposed by the tension spring. When the coolant temperature falls below the opening level, the tension spring exerts pressure on the valve and restores the sensitive element to its original position, thus closing the cooling circuit.

The bypass valve on some thermostats has the function of obstructing the flow of the coolant to the radiator and diverting it through a secondary tube. This mechanism allows thermal balance to be reached more quickly.

Thermostats with plastic or aluminium bodies

These thermostats are integrated in a plastic or cast aluminium body connected to the cooling system by connectors and/or couplings.

Facet also produces thermostats with very complex shaped housings and fitted with engine cooling system connectors and temperature control sensors.

Map-controlled thermostats

This type of thermostat provides broader and faster operation than traditional thermostats. In addition to the mechanical function of the wax bulb, map-controlled thermostats have an electric heater integrated into the sensor. The heater, controlled by the vehicle’s engine control unit, changes the temperature of the coolant according to the speed and load conditions of the engine. This temperature regulation is governed by a map stored in the engine control unit.

In partial load conditions (urban cycle), the thermostat stabilises the engine at a higher temperature to obtain benefits such good reaction kinetics, lower emissions and reduced friction (with a consequent reduction in consumption).

In full load conditions, the heater is activated to allow lower running temperatures and increase the power of the engine.