The thermostat on a vehicle is the component that opens and closes to allow coolant to flow in and out of the engine. A thermostat remains closed while the engine is still cold; this keeps coolant trapped inside the radiator. Once the coolant engine reaches proper operating temperature, typically 180 to 200 degrees, the thermostat opens up and allows the coolant to flow through the engine. Sometimes the thermostat fails and must be replaced; there are four main reasons for failure: overheating, sludge, defect and age.
The thermostat senses the heat of the engine, and then uses that heat to open and close a valve. If the engine overheats for any reason, this can cause the thermostat to fail. The components inside the thermostat are only designed to handle normal operating temperatures, and severe overheating may damage said components.
Just as with any automotive fluid, over time the coolant can become contaminated and start failing. A common problem is the congealing of the coolant into a thick sludge-like material. This sludge can get inside the thermostat and cause the flow to be restricted or even cut-off altogether. The sludge also prevents the thermostat from taking accurate readings, causing delay in opening or closing, which leads to over-cooling or overheating.
Just as most automotive parts, the thermostat is mass produced in a factory. Before a thermostat is released for sale, it is inspected for proper operation. The human element plays a large part in the role of inspecting the thermostats and some problems are missed. These minor defects can lead to a thermostat being defective straight from the box, or failing shortly after installation.
Age is another common reason for failure. The thermostat is constantly heated and cooled, and after a while the internal components can simply wear out. This usually occurs slowly and almost unnoticed, but the temperature in which the thermostat opens begins to get higher and higher, until one day a complete overheat occurs. Unfortunately, this is just the life-cycle of mechanical components.
Installing a thermostat can be tricky at times, and even some professionals install them incorrectly. One common problem is to install the bleeder valve -- commonly called the jiggle valve -- upside-down, not allowing air to bleed from the cooling system. Another improper installation problem is installing the thermostat backwards. This puts the part of the thermostat that senses the temperature toward the radiator and not in the engine. The thermostat will still open and close, but it causes a long delay because the heat must travel farther before reaching the thermostat.
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