ABC's of Thermostats

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

Additional information