ABCs of Thermostats

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
• Flange
• Spring
• Frame

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

Thermostat Temperatures

• 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.

Thermostat Failures

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