How To Know if a Car Thermostat is Faulty

A car's thermostat is one of the many parts of an engine that everyone seems to know about, but few actually understand - until it becomes faulty.  It is only when steam pours out from the engine compartment and a bright red TEMPERATURE light illuminates the dashboard that a driver's attention is suddenly directed to the relatively minor and often neglected car thermostat.

A car thermostat is a valve that regulates the flow of coolant inside your engine.  The combustion of fuel creates a great deal of heat.  To prevent damage to the engine, coolant (a solution of water and anti-freeze) is pumped through the engine to absorb the heat and carry it away.  The hot coolant is pumped under pressure through the radiator where the heat is dissipated as cool air is blown through the radiator core.  Until the coolant reaches the optimal operating temperature, the thermostat remains closed and the coolant circulates through the engine without passing through the radiator. 

When the coolant reaches the proper temperature, the thermostat opens and permits the coolant to circulate through the radiator to maintain that temperature.  This is also the time that coolant is directed to the vehicle's heater core.  The heater core is a small radiator, located in the passenger compartment of the car.  When the thermostat opens, heated coolant circulates through the heater core where a fan blows the radiated heat throughout the passenger compartment.  Until the thermostat opens, there is no heat in the passenger compartment, a condition all too familiar to those in colder climates.

The thermostat itself is a very simple device that resembles a child's gyroscope.  At the bottom is a heat sensitive pellet.  Above it is a spring-loaded metal plate that acts as a valve.  As the coolant temperature rises, the pellet expands.  As it expands, it draws down the metal plate, allowing coolant to pass through the thermostat and into the radiator.  The default position of a car thermostat is closed. When a car thermostat is faulty, it will either open partially or not at all.  If your car has a temperature gauge, a sign that the car thermostat is faulty will be that the temperature runs hotter than normal.  In a car with a dashboard light rather than a gauge, an indication of a faulty car thermostat will be steam from the engine compartment and the lack of heat in the passenger compartment. 

These symptoms are also consistent with a low coolant level.  If adding coolant does not eliminate the condition, the problem is coolant circulation, and that can be attributable to a faulty thermostat, water pump or drive belt.   Newer, innovative thermostats are designed to fail in the open position to prevent the car from overheating.  Continuing to drive with an overheated engine can do significant damage and result in costly engine repairs.  Stop an overheated car and allow it to cool before seeking service.


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