The thermostat is one of the more important components of an automobile. Think of the cooling system as two parts; the engine assembly and the radiator.
In the first part, all the heat is generated. In the second, heat is removed. The thermostat is a valve between the two.
Engines run best at a certain temperature; when they are cold, they run very poorly and if they get too hot, problems arise. Various sensors and heat management strategies are employed but at the core is the thermostat. It holds heat in the engine until operating temperature is achieved; then, it opens and allows coolant to circulate to the radiator and maintain that temperature.
Usually the thermostat is located where the upper radiator hose meets the engine assembly and most of the time (though not always), it is fairly accessible.
What’s the Price of a Car Thermostat?
For some examples of the thermostat replacement cost on some common vehicles, we are using $100 an hour as a labor rate:
For a 2010 Subaru Impreza with a 2.5-liter engine, the labor time is 0.7 of an hour. An OE replacement part is about $22 and a Gates part is about $11. Adding about $20 for coolant, the total cost for the job is about $112 using a factory replacement part and about $101 using aftermarket.
For a 2003 Chevrolet S10 Blazer with a 4.3-liter engine, the labor time is 1.1 hours. An OE replacement part is about $14 and a Stant part is about $5. Adding about $13 for a gallon of coolant, the total cost for the job is about $137 using factory replacement parts and about $128 using aftermarket.
For a 2012 Toyota Tundra with a 5.7-liter engine, the labor time is 1.4 hours and an Aisin replacement part is about $32. Adding about $18 for coolant, the total cost is about $190 to complete the job.
For a 2000 Volkswagen Passat with a 2.8-liter engine, the labor time is 4.9 hours (the thermostat is behind the timing cover, requires timing belt removal). An OE car thermostat’s price is about $41 and a Hella replacement part is about $12. With about $15 in coolant figured in, this makes the job about $546 using factory parts and about $517 using aftermarket parts.
As far as cost savings, sometimes, the most economical time to replace a thermostat is before it fails. There is no regular scheduled replacement interval for the part and they don’t normally wear out from use.
However, if an engine or cooling system is being serviced for other reasons, then, installing a new thermostat during related work can be good preventative and inexpensive maintenance.
Other Things That May Come Up
Given that replacing the thermostat involves draining about a gallon of coolant, the most common service that goes along with thermostat replacement is a cooling system flush. With the system open and partially drained, it is fairly easy to complete. If the cooling system in any way indicates a need for a general scheduled service, it will probably be recommended.
Many modern cooling systems are complex internally and prone to trapping air. In many cases, normal service procedures require the system to be purged of air with a vacuum device after draining.
If a shop lacks the equipment, it can take a long time to work the air out of a cooling system and sometimes, components (such as the heater) won’t work normally for a while. In the worst cases, a car may continue to run hot until it is fully purged.
The thermostat is easily damaged or rendered unreliable by an overheating event. This means that it’s not always simple to distinguish the cause and effect.
Sometimes, a thermostat can be diagnosed and replaced. Once it is working, another underlying cause for engine overheating is found. After replacement, attention should be paid to the engine temperature and other components to verify that everything is working as it should.
How Thermostats Fail
A thermostat is basically a valve operated by a rod and the rod is set in a cylinder filled with wax. When the temperature of the cylinder reaches a certain point (180 to 190 degrees typically), the wax melts and expands; pushing the rod up and opening the valve.
Mechanical failures are possible as deposits sometimes build up and prevent the valve from closing fully. The rod can also migrate from a centered position with wear and no longer opens or closes fully, or sticks in one position.
Another type of failure that can result from overheating is if the wax expands and over-pressurizes the cylinder it resides in due to excessive temperatures and the liquid wax then leaks past its seals. If even a small amount of wax is lost, the valve won’t open fully or not at all.
It is fairly easy to test a thermostat by placing it in water that is being brought to boil. By measuring the temperature of the water at the point where the valve opens and comparing that to the desired engine operating temperature, the valve can be verified as operational or not.
In practice, this is very seldom done. Most thermostats are much cheaper than the labor time involved in bench testing them. The normal recommendation, if one is suspect, is to replace it almost as a maintenance item.