A thermostat is the key component of the engine cooling system. The cooling system keeps the engine from overheating. The system is filled with liquid coolant (water plus antifreeze) and is connected into a loop with a radiator.
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| A Toyota thermostat |
A conventional thermostat is a simple temperature-controlled two-way valve that opens at a specified temperature.
In most cars, a thermostat is installed on the engine and is connected to the upper or lower radiator hose.
The coolant flows through the engine where it absorbs the heat and into the top of the radiator. The air passing through the radiator cools down the coolant as it flows down the radiator.
One or two electric fans attached to the back of the radiator turn on when needed to force more air through the radiator. In Nigeria, we often mess this up and "connect am direct to ignition". A water pump pulls the coolant from the radiator and moves it back into the engine.
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| A car cooling system |
The normal operating temperature of a modern engine is between 194°F (90°C) and 221°F (105°C). The job of a thermostat is to help the engine warm up faster and maintain the engine temperature above the minimum operating temperature (194°F or 90°C).
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| Ford 2.3L EcoBoost engine. |
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| Mazda 2.3L turbo DISI engine. |
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| Dodge Charger SRT Hemi engine. |
How A Thermostat Works:
When the engine is started cold, the thermostat (the main valve) is closed; there is no coolant flow through the radiator. The smaller by-pass valve is open and the coolant circulates only within the engine and through the vehicle heating system.
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| A car thermostat |
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| Car thermostat inside the housing. |
As the engine warms up closer to the operating temperature, the thermostat gradually opens up, allowing coolant flow through the radiator. In cold weather, if the engine temperature drops close to the lower limit of the operating range (194°F or 90°C), the thermostat closes again.
Symptoms Of A Bad Thermostat
These symptoms can be divided in two types:
Thermostat stuck open: When the thermostat is stuck open, the engine temperature drops below normal when driving, especially on the highway in cold weather. A stuck-open thermostat can also cause lack of heat from the heating system. This may not be very obvious in our climate. The Check Engine light may come on too.
Thermostat stuck closed: When the thermostat is stuck closed, the engine might overheat. Besides these two cases, the thermostat may also need to be replaced if the thermostat housing is leaking coolant.
A thermostat could stick open if the return spring breaks or debris prevents the thermostat from fully closing: Consequently, This allows a steady flow of coolant to the radiator; over cooling the engine.
This results in poor warm up and heater performance; increased engine emissions and reduced fuel economy.
This is much worse when you just take the thermostat and connect the fans directly to the ignition like our Nigerian mechanics like Sule and Oga Monday always do.
A thermostat could stick closed if the wax element has been damaged by overheating or corrosion: possible overheating from loss of coolant; a defective electric cooling fan or fan clutch. Also, This failure prevents the flow of coolant to the radiator; possibly overheating the engine and causing damage.
When an engine overheats it’s always a good idea to replace the thermostat. Above all, The consequences of an “open failure” may be less catastrophic than those of a “closed failure”.
How To Test A Thermostat
When dealing with any cooling system problem, the coolant level is checked first.
This is for me the simplest of several available ways, by checking coolant flow:
- Start off with a cold engine.
- Remove the radiator cap.
- Start up the engine and let it idle.
- Verify that the coolant is not flowing.
- You can check this by looking through the radiator filler neck.
- The coolant should not be flowing.
Consequently, the coolant has not reached a high enough temperature to open the thermostat.
So, If you see the coolant flowing; you have a thermostat stuck in the opened position. As a result, you need a new thermostat.
If the coolant is not flowing let it run for about 20 minutes to reach operating temperature.
About this time, you should see the coolant through the radiator filler neck beginning to flow.
Consequently, The coolant begins to flow because it has reached a high enough temperature to open the thermostat.
So, If you don’t see the coolant flowing; you have a thermostat stuck in the closed position. As a result, you need a new thermostat.
If the coolant begins to flow and the temperature stays normal, your thermostat is fine.
In modern cars, the operation of the thermostat is monitored by the engine computer (PCM). In most cases, if a thermostat is stuck closed or open, the Check Engine light would come on.
Thermostat Replacement
A thermostat is also often replaced together with a water pump, especially if they are located in the same area.
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