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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

Quality Policy

Customer Satisfaction: To provide products and services which entirely satisfy the requirements as defined by the customer. Each of us is, therefore, committed to excellence and to assist one another to pursue continuous improvemrnt.

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Phone: (765) 825-3121
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