The Purpose of a Radiator Coolant Overflow Tank

How a Coolant Overflow Tank Works and How It Can Prevent Rust Formation Inside the Cooling System

The cooling systems of a car may seem to work fine without any additional equipment attached to it but that will not save the cooling systems of your car from the environmental damage like rust due to the way coolant and the radiator works. Rust can be formed inside the cooling systems of your car due to the air getting into the radiator after the engine has cooled down. When the coolant cools after the engine has been turned off, it contracts in volume which will result in a drop of temperature and pressure inside the radiator allowing the air to enter the radiator through the radiator cap. That air will result in the formation of rust in the cooling system when it comes into contact with the metal in the cooling system.

When the rust has formed inside the cooling system of your car, you will have no choice but to flush out the system and replace the coolant but a radiator coolant overflow tank can prevent this problem before it even forms thus saving you the expense of frequent coolant replacements and possibly the expensive cost of the installation of a new cooling system.

How a Radiator Coolant Overflow Tank Works

The radiator coolant overflow tank is fairly simple device but it can save you a lot of money by preventing rust formation in the cooling system of your car. When the engine is running at its optimum performance, it needs the engine coolant to keep it within the operational temperatures. The engine coolant absorbs the heat from the engine. When this coolant heats in the radiator, it begins to expand causing an increase in the pressure in the cooling system of that vehicle. When the pressure of the coolant in the radiator surpasses the pressure cap setting, the coolant passes the pressure cap seal and flows into the radiator coolant overflow tank through the overflow tube.

When the engine has been turned off, the coolant will start to cool down as well. As it cools down, it will contract in volume as well and this decrease in volume will result in a vacuum because of a drop in pressure and due to this vacuum, a vacuum valve in the pressure cap opens allowing the coolant which was flown into the radiator coolant overflow tank back into the radiator.

In the vehicles with no radiator coolant overflow tank, the air is sucked into the radiator instead of coolant which results in the formation of rust. So installing an overflow tank in your car is a good way to not only maintain the cooling systems of your car but to save repairs and replacement expenses in the long run as well.

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  1. Great article. Thank you. Quick question; if the coolant overflow tank cap doesn’t have a good seal, or is missing, can this cause engine to slowly overheat? Is the coolant overflow tank part of the whole cooling system pressurized system? Thanks

    • The coolant tank overflow is non-pressurized on almost all modern vehicles (if the cap is plastic, it’s non-pressurized). That said, as coolant heats up it expands. So if you don’t have a tight seal, coolant can leak out and ultimately cause you to be low on coolant which would cause overheating. Check your coolant level and if you notice the level decreasing, get that cap replaced asap.

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