Most gasoline and diesel engines include a water-based cooling system to remove excess heat caused by combustion. A coolant circulating through the walls of the engine picks up heat and carries it into the fins of a radiator where moving air helps dissipate the heat.
Cooling systems are simple compared to engines. They include the water jacket, a network of passages though the engine block; the radiator and cap; a fan; a water pump; hoses and the thermostat. Though it's a mechanically simple device, the thermostat serves an important function, regulating the temperature of coolant and engine.
A thermostat sits across the opening where coolant enters the block. When the engine is cold, the thermostat closes off the opening to keep water isolated in the block. As the engine begins to generates heat. The thermostat remains closed until coolant inside the block reaches the operating temperature at which the engine performs its best. Then the thermostat opens to allow circulation of coolant from the remainder of the system. It opens wider as the coolant temperature increases: the more coolant that passes through the block and the radiator, the more effectively the system removes unwanted heat. When operating temperatures change, the thermostat can adjust to maintain the coolant's temperature within a narrow set range.
There might be steam coming from under the hood or an "idiot light” may suddenly illuminate on an instrument panels that has no temperature gauge. The “Check Engine” light may also come on; if you have access to a code reader you can diagnose the problem from the computer. If you suspect the thermostat is at fault, they are relatively easy to test.
2) Inspect the thermostat. If it is stuck in the open position, replace it.
3) If the thermostat is not stuck open, test it further.
4) Fill a pan with sufficient water to cover the thermostat. Place a thermometer in the water and place the pan on a heat source.
5) Slowly raise the temperature if the water. The thermostat should not change until the water reaches about 187°F (86°C), at which temperature it should start to open. It should be fully open at 195°F (90.6°C). Check the manufacturer's specifications for the gap created when it's completely open.
6) If the thermostat is still closed when the water starts boiling, it is definitely faulty and you must replace the thermostat and gasket. If it opens at too low a temperature, it must also be replaced.
• The 195-degree temperature is most common, but it's not a constant. The thermostat's operating temperature is usually stamped on its housing. If there's doubt, consult the vehicle’s shop manual online or at your library.
• Never run an engine without a thermostat, because the system's water pump is calibrated to the "bore" of the opening with the thermostat in place. If the opening to the block is not partially blocked by the housing, the coolant can move through the system too fast to allow heat transfer, and allow the engine to overheat.
• A thermostat can't be repaired; it must be replaced.
Cooling systems are simple compared to engines. They include the water jacket, a network of passages though the engine block; the radiator and cap; a fan; a water pump; hoses and the thermostat. Though it's a mechanically simple device, the thermostat serves an important function, regulating the temperature of coolant and engine.
How a Thermostat Works
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| A typical thermostat. |
How Thermostats Fail
When it reaches the end of its useful life, a thermostat usually sticks in the open position, allowing coolant to circulate through the system continually. When this happens, an engine is unlikely to reach its optimum operating temperature, which both increases wear and erodes fuel economy. In cold weather the heater, which is in most cars part of the cooling system, will function poorly or not work at all. More rarely, the thermostat will get stuck in its closed position. In this case, coolant in the system won’t circulate through the engine block, causing the engine to overheat.How to Know Your Thermosat's Gone Wonky
If the vehicle has a temperature gauge, the needle should rest within the marked range while the engine is running and the vehicle is moving. If it goes above the range (hotter), the thermostat may be stuck closed. If it stops below the range (cooler), it is probably stuck open. The heater may not blow any hot air.There might be steam coming from under the hood or an "idiot light” may suddenly illuminate on an instrument panels that has no temperature gauge. The “Check Engine” light may also come on; if you have access to a code reader you can diagnose the problem from the computer. If you suspect the thermostat is at fault, they are relatively easy to test.
Testing a Thermostat
1) Remove the thermostat: the procedure varies from vehicle to vehicle, but they are generally at the engine end of the top radiator hose.2) Inspect the thermostat. If it is stuck in the open position, replace it.
3) If the thermostat is not stuck open, test it further.
4) Fill a pan with sufficient water to cover the thermostat. Place a thermometer in the water and place the pan on a heat source.
6) If the thermostat is still closed when the water starts boiling, it is definitely faulty and you must replace the thermostat and gasket. If it opens at too low a temperature, it must also be replaced.
Notes and Tips
• If you use a saucepan and a candy thermometer from your kitchen in the test, clean them thoroughly before using them to prepare food: the ethylene glycol found in coolant is poisonous.• The 195-degree temperature is most common, but it's not a constant. The thermostat's operating temperature is usually stamped on its housing. If there's doubt, consult the vehicle’s shop manual online or at your library.
• Never run an engine without a thermostat, because the system's water pump is calibrated to the "bore" of the opening with the thermostat in place. If the opening to the block is not partially blocked by the housing, the coolant can move through the system too fast to allow heat transfer, and allow the engine to overheat.
• A thermostat can't be repaired; it must be replaced.
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