Cooling / Tech Articles

Mailbag: How Do Thermostats Work Within a Cooling System?

(Image/Summit Racing)

In a liquid cooled engine, the thermostat is a valve installed between the water pump and the radiator. It controls when the radiator starts to cool the engine. It’s job is to help the engine reach operating temperature quickly.

How does it work?

A cylinder in the thermostat is filled with a special wax. A rod is set in the wax and attached to the valve.

  • As the wax heats up, it melts and expands.
    • This forces the rod to move and open the valve.
  • As the wax cools, it contracts.
    • Spring pressure moves the rod back and closes the valve.

When the engine is cool, the thermostat is closed. It blocks flow to the radiator so coolant only circulates through the engine.

When the coolant reaches the melting point of the wax, the thermostat starts to open. This is known as the “opening temperature” of the thermostat.

The open valve allows coolant to flow through the radiator. Then, the radiator and the cooling fan can do their jobs. As the coolant temperature drops, the thermostat starts to close.

This process continues as the engine is running. The thermostat opens and closes, based on the temperature of the coolant.

How does it affect performance?

It’s a common belief that the thermostat controls the operating temperature of the engine. This is only partly true. The thermostat sets the bottom of the operating range. The top of the operating range depends on the combined cooling capacity of the radiator and fan(s).

Engines are designed to run most efficiently within a certain temperature range. Usually, this is somewhere between 180-200 degrees. The thermostat really prevents “over-cooling.” This means the engine warms up faster and stays warm.

Most engine wear occurs when it’s cold. Without a thermostat, it would take a long time for the engine to reach operating temperature. This would cause excessive wear to bearings and other moving parts.

This is another in a series of weekly Q&A Mailbag sessions with Summit Racing‘s tech department, in which there are hundreds more. Click here to see them all.

Tags: , , ,


  1. Pingback: Mailbag: How Do Thermostats Work Within a Cooling System?

  2. A lot of people are confused about the”Thermodynamics” of a Thermostat. A Thermostat is very important. When I lived and Worked in Florida the most common thing I heard about a Thermostat is you don’t need one, it doesn’t get that cold down here, along with you don’t need Antifreeze, {Thank God they finally put antifreeze/coolant on the jugs} I always recommend A 195*thermostat, as long as you have a good/clean cooling system. It keeps the Engine in good Balance across the scale. It also keeps the Internal parts of the Engine clean.(ever pull valve covers/intake manifold and find that disgusting MUCK?) 9 out of 10 times the cause is a missing/ stuck open thermostat or one that’s too cool.(160*)
    I’ve also have seen Engines Overheat due to lack of a Thermostat. Coolant was passing too fast thru the Radiator for proper heat transfer. I’ve been building Engines(not MOTORS!) for 40+ years(Gas&Diesel) and never had a problem with a 195* thermostat.[you’ll notice a big difference between a 180* vs 195* thermostat,yes that 15 degrees is crucial!]

  3. I too have worked on engines for 50 plus years. Never had a strong interest in motors, just engines.

  4. My 1969 GTO just had the motor refresh and it running 220 to 230. AND BEFORE I HAD THE MOTOR DONE IT RUN AT 190.WE CHANGED The thermostat put a
    180in. Everything is new

  5. Thanks for sharing keep it up! Our site

  6. Brielle Luna says:

    Will it be working fine being with a stock cooling system?

  7. Cool! Thanks a lot for all the tips!

    apply here

Leave a Reply

Your email address will not be published.


This site uses Akismet to reduce spam. Learn how your comment data is processed.