How a Radiator Coolant Overflow Tank Works (Visual Image)

The overflow tank may seem like a simple part, but it is actually quite cleverly designed. This is how a radiator overflow tank works

Coolant Overflow Tank (2)

This is one of the most in-depth guides you’ll probably ever come across for your vehicle’s cooling system. We’ve decided to not only explain how your overflow tank works on its own but also how it works in unison with other engine components.

If you’re in a rush then feel free to skip down to the relevant subheading that you’re after, but just know we’re very upset with you for doing so. 😉

The cooling system 

Cars Cooling System

As you’re probably already aware, your engine gets incredibly hot as you drive. So hot in fact, that it can cause parts of your exhaust to glow bright orange! 

So your car needs a system in place to provide an efficient means of cooling. It needs a system in place to take away the heat that’s generated from the engine to stop it from overheating. 

This is where your cooling system comes into place. Your vehicle’s cooling is simply designed to cool the engine by carrying its generated heat away. As your engine turns over and gets hot, the coolant that passes through and around your engine steals this heat away and carries it to the front of the car where it’s cooled off by the radiator and cooling fans. 

This amazing piece of engineering is designed to keep your engine operating at a constant 90 degrees centigrade, whether you’re flying along the motorway, or stuck in traffic. It really does do a brilliant job of this too. 

Coolant Overflow Tank Function

How a Radiator Coolant Overflow Tank Works

The radiator coolant overflow tank works by filling the tank with coolant when the cooling system’s pressure is too high and withdraw coolant from it when there is under pressure in the system. The coolant is transferred back and forth between the tank and cooling system depending on the pressure.

In order for this cooling system to work effectively, it needs to be sealed. This prevents any leaks as well as any dirt or debris from entering the cooling system. 

This design works fine, as long as your coolant doesn’t get too hot. As your coolant solution heats up, it expands. In a sealed system, however, there’s no room for the liquid to expand. This can cause a problem as an over-pressurized system can cause your liquid to boil faster and to burst through any weak points on the system.

This is the reason why we need to have an overflow tank as the pressure inside the coolant pipes increases due to the coolant solution being heated and therefore expanded. The expanded coolant needs somewhere to go. Any liquid that no longer fits inside the primary cooling system is now forced into the overflow tank, where it’s stored until the engine coolant temperature reduces. As the coolant begins to cool, it contracts and increases the amount of room once again inside the cooling system. This contracting of the coolant creates a negative pressure which draws the extra coolant stored in the overflow tank back into the primary cooling system. 

Therefore, your overflow tank is used as an expansion vessel to allow the coolant mixture to expand and contract as it warms and cools, without over pressurizing and damaging the cooling system. 


Refill Coolant E1611338886692

Your coolant is made up of a water and antifreeze mixture. Water is great as it’s able to do the job and is readily available in case of an emergency breakdown. However, antifreeze is also added, just to help improve the water’s qualities. As the name suggests, it helps prevent water from freezing in lower temperatures when the vehicle isn’t in use by lowering its freezing point. But, it also stops the water from boiling and increases its boiling temperature by a few degrees. 

Built into your vehicle’s coolant mixture is also a rust protection formula, which is designed to help prevent the internals of the cooling system from corroding and rusting from the water.

What’s great about this cooling system is how adaptable it is. If your vehicle has an EGR valve fitted, then it will often be cooled using your engine’s coolant. The same goes for some turbos also. 

The heat that gets stored into this coolant mixture can also be used to heat the inside of your vehicle through a heater matrix. Making it a very efficient heater as you’re using waste heat to warm yourself. 

Water pump 

Water Pump Bad E1609789871700

The only problem with this system is the heat that it gathers from the engine, which has to go somewhere. But how does the coolant move from one section of the engine to another? A water pump. Usually, water pumps will be driven either from your auxiliary drive belt or your cambelt. There is a pulley with a propeller on the back of it that pushes the coolant around the engine and radiator pipes. 

The cooling system is fully sealed, so the water pump doesn’t have to work too hard in order to circulate the coolant. 

Your water pump is a major component in preventing your coolant from overheating. It pushes the hot liquid from inside the engine to the radiator at the front of the engine where the liquid gets cooled off by either the fans or by incoming air. This cool liquid is then ready to be re-circulated around into the engine to carry more heat away, and then the process repeats itself constantly whilst the engine is running. 

If for whatever reason your water pump was to stop working, then your coolant would become overheated and start to pressurize and boil. This would then lead to your engine overheating as your vehicle will drain off any coolant that is boiling, which leaves you with less coolant to cool your engine with. 

Radiator cap 

Radiator Cap

The radiator pressure cap is the unsung hero of your cooling system and works in unison with your overflow tank. Many people think that your radiator cap is simply a screw-on-lid for your radiator, but it’s so much more. 

Inside your radiator cap is a spring-loaded valve. As the coolant temperature increases and the pressure inside causes the coolant to expand, the valve inside your radiator cap opens to allow the excess coolant to flow into your overflow tank. This valve allows your cooling system to stay pressurized but will help prevent the system from over pressurizing and thus overheating. 


So, there you have it. A clear guide on not only how your overflow tank works, but also why you have one and how it works with other engine cooling components. 

Your vehicle’s cooling system truly is an amazing piece of engineering that’s been carefully designed to be efficient and effective at what it does. 

Hopefully, this guide has answered any questions that you may have had, and please feel free to refer back to it whenever you need to.

Category: Coolant
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Written by:

Magnus is the owner and main author of MechanicBase. He has been working as a mechanic for over 10 years, and the majority of them specialized in advanced diagnostics and troubleshooting. Certified Automotive Diagnostic Technician.