Monday, July 28, 2014 Detailed Auto Topics
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Most folks realize an engine thermostat can cause a vehicle to overheat. Did you know thermostats are also one of the leading causes of poor fuel milage?

Engine thermostats perform three major functions

  1. Help an engine to reach operating temperature more quickly
  2. Help regulate engine temperature to prevent overheat
  3. Restrict flow to promote more even cooling

A cold engine waste fuel

Most modern engines operate close to 200 degrees Fahrenheit. Below this operating temperature is considered cold and the engine needs more fuel to run properly. On older carbureted engines, a choke was used to add more fuel. With fuel injected engines the pulse-width of the fuel injector is increased. Increasing pulse-width also drastically decreases fuel milage. This is why it is desirable to reach operating temperature as quickly as possible.

how an engine thermostat regulates coolant flow

When the engine is cold the thermostat blocks flow to the radiator and opens the bypass circuit. In this mode the water pump circulates coolant only inside the engine. The coolant is prevented from flowing through the radiator where it would be cooled.  Instead coolant flows through the bypass and back to the water pump. The effect is a quick increase in engine temperature.

a closed thermostat bypasses the radiator

How an engine thermostat works

The thermostat is a temperature controlled valve. The spring loaded valve is attached to a small cylinder filled with thermal wax. As the engine warms up, coolant transfers heat to the wax-filled cylinder. The heat causes the cylinder to expand and pushes down on the spring loaded valve.

Parts of a typical engine thermostat with a bypass valve

When the thermostat opens flow to the bypass port is blocked and flow to the radiator is enabled. The water pump now pushes coolant through the cylinder head and out to the radiator, where the heat is removed. Lower temperature coolant leaves the radiator and circulates back to the water pump completing the cycle.

coolant flow with the engine thermostat open

If the temperature begins to fall below the limit set by the thermostat the flow will be restricted until it rises to the proper level. In a well maintained cooling system the thermostat will last for many years.

stuck thermostat from a poorly maintained cooling system

As coolant ages, the additives that prevent corrosion are depleted. This causes a huge amount of damage to the system and can cause thermostats to stick. If the thermostat sticks open, the engine runs too cold and waste fuel.
Engine overheating

a thermostat stuck closed will quickly overheat the engine

When a thermostat sticks closed the engine will overheat. Overheating causes cylinder heads to expand greatly. This has the effect of crushing the cylinder head gasket and can crack the head[s]. Pistons also expand and gall the cylinder walls. Overheating, even for a short while can severely damage the engine.

A non-working thermostat should be replaced with the exact temperature replacement specified for the vehicle. Removing the thermostat will NOT increase cooling. An engine without a thermostat will flow too much coolant and can erode the radiator and heater core.  The increased velocity of the coolant flow can easily wear through the thin metal tubes. This is far worse in a dirty system, full of abrasives, from lack of care.  Too much flow can also hamper cooling, as the coolant does not remain in the radiator as long and may give off less heat.

Testing a thermostat

Thermostats can easily be tested with simple household items, if they are suspected of causing a problem. First inspect the thermostat at room temperature. It should be fully closed. Heat a container of water to a temperature 25 degrees less than listed for the vehicle. When the temperature is reached, submerge the thermostat, using a piece of wire. If the thermostat is good, it should not open.

simple test for an automotive engine thermostat

Next bring the temperature of the water to the temperature that is listed for the thermostat (i.e., 195 degrees on most vehicles.) Again submerge the thermostat and it should open immediately and fully.

Any thermostat that fails either test should be replaced. It is also a good idea to replace the thermostat when other cooling system work is being done that requires it to be removed. This is most important on vehicles with higher milage and less important on low milage engines.

If your fuel milage is significantly decreased or if your cooling system has not been properly maintained, the engine thermostat should be checked. This inexpensive component can save literally thousands in repair.





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