Purpose of a Thermostat
The thermostat has two important jobs:
- Accelerate engine warm-up: By blocking the circulation of coolant between the engine and radiator until the engine has reached its predetermined temperature
- Regulate the engine’s operating temperature: By opening and closing in response to specific changes in coolant temperature to keep the engine’s temperature within the desired operating range
Parts of a Thermostat
The basic parts of a thermostat are:
- Heat motor, which includes a valve attached to a piston that is embedded in a special wax
Some thermostats also have a disk at the base that closes a bypass circuit inside the engine as it opens the radiator circuit.
- The bypass circuit circulates coolant inside the engine so that hot spots can’t form when the radiator circuit is closed.
How a Thermostat Works
- When the engine is cold, the thermostats is normally closed; restricting flow to the radiator allowing the engine to “warm up”
- As the engine warms, the increase in heat causes the wax to melt and expand, pushing against a piston inside a rubber boot
- This forces the piston outward, opening the thermostat so coolant can start to circulate between the engine and radiator
- As heat increases, the thermostat continues to open until engine cooling requirements are satisfied
- If the temperature of the circulating coolant begins to drop, the wax element contracts; allowing spring tension to close the thermostat, which decreases coolant flow through the radiator
- Thermostats have a “rated” temperature such as 180F or 195F
- This is the temperature the thermostat will start to open, give or take 3 degrees
- The thermostat fully open about 15-20 degrees above its rated temperature
- Many thermostats have a “jiggle pin” or “check valve” that allows trapped air in the cooling system to pass through the thermostat and be released from the system.
- If a Stant thermostat does not have a jiggle pin, it will have a “bleed notch” or other method of removing air from the system.
Why is a Superstat Super?
Superstat premium thermostats have:
- Heavier frames and springs
- Larger heat motors
- Patented Weir valves
The unique Weir valve provides a higher flow of coolant to the radiator than any other thermostat on the market
- The early flow of coolant minimizes the “cycling” of temperature allowing the engine to run a steady temperature, resulting in higher efficiency.
A thermostat fails “open” if the return spring breaks or debris prevents the thermostat valve from fully seating or closing; allowing a steady flow of coolant to the radiator, overcooling the engine
- This results in poor warm up and heater performance, increased engine emissions and reduced fuel economy
- An engine should never be operated without a thermostat, even in extreme temperatures
A thermostat will fail “closed” if the wax element has been damaged by overheating (from loss of coolant, a defective electric cooling fan or fan clutch) or corrosion
- This failure prevents the flow of coolant to the radiator, possibly overheating the engine and possibly damaging the engine
- When an engine overheats, it’s a good idea to replace the thermostat because it could be possibly damaged
Can a thermostat fail safely?
There is no such thing as a thermostat that will fail in a “safe” position.
- A thermostat will fail in either a closed or open position.
One brand claims it fails in a safe position
- This brand locks itself open when it is a full stroke open position, basically breaking the thermostat.
- It does not spring open if it fails in a closed position.
OE & Alternate Temperatures
OE recommended temperatures should be used in nearly all situations
Alternate temperature thermostats are available and can be used in some applications
- Alternate temperatures are often applicated for older, pre-1995 vehicles where they can be used without causing problems
- Alternate temperature are not applicated for most newer vehicles
- Using an alternate temperature thermostat in a newer vehicle would require changing computer setting and possible additional modifications