Everything You Need to Know About Radiator Overflow Tank

The current trend of engine downsizing while pumping out more power than before has a few knock-on effects. The higher that rated power the higher the generated heat. A large portion of this goes through the engine block, and another exits the car in the form of exhaust gas. These parts need to be made of materials that not only hold their own against high temperatures but also high pressures. In short, modern engines may be exemplary feats of advanced engineering, but they come under a lot of stress.  

Maintaining that rise in heat is done by the vehicle’s cooling system and to a certain extent the engine oil. Combined, these two prevent parts from literally melting. And the whole system works as a calibrated unit. Any part that is defective can lead to overheating, or complete engine failure, and a costly repair bill that you want to avoid.  

Cooling System Parts 

Cooling System Parts Radiator Overflow Tank
Source: cartreatments.com

Cars maintain heat by circulating coolant through vital engine components. Coolant collects excessive heat and redirects it to the radiator where it is subjected to further cooling from oncoming air and high-speed fans. Helping out are water pumps, that provide the needed pressure to push coolant through a system of connected hoses and pipes, a thermostat that opens and closes to let coolant flow depending on engine temperature, and a radiator overflow tank to collect expanding coolant as pressures in the sealed system increase. Other parts, like radiator caps, prevent coolant loss by regulating coolant system pressure, while coolant temperature sensors inform the car’s ECU about how hot the engine is so that this can control the amount and timing of the fuelling.  

While overflow tanks are simple in design and working principle, they are a key part without which a modern engine just won’t be able to work.  

What are Radiator Overflow Tanks? 

full Radiator Overflow Tanks
Source: rxmechanic.com

Coolant expands when it gets hot, and a radiator overflow tank allows the sealed system some wiggle room in storing coolant that is trying to make a run for it. As such, the tank ensures that there is no coolant loss or leaks and serves as a simple expansion tank. And it ensures that the pressure within the entire cooling system remains constant. The amount of coolant in the tank can change as engine temperatures vary and this is to ensure that the engine is working at optimal working temperatures.  

How it Works 

There are two basic types here, overflow and expansion tanks. Overflow (or recovery) tanks don’t have their own cap, instead, this part is integral to the radiator. As coolant gets hot and expands, a valve in the radiator cap opens and allows coolant to flow through hoses into the overflow tank, where it is stored until temperatures in the cooling system drop.  

Expansion tanks have their own cap, so function a little differently. The system here is pressurised, meaning coolant is constantly being cycled between the radiator and tank. Hot coolant expands from the tank and fills the top half of the radiator through a separate by-pass or heater hose. This a more complex system, but also more effective. It also means that the tank will often be placed higher in relation to the radiator.  


Plastic is the material of choice as it can adequately handle the rise and fall in engine and coolant temperatures. To make things easier when checking on coolant levels, the tank is also translucent, so the coolant is easy to see. One downside of plastic in factory tanks though is that it won’t perform in engines that have been modified for more power, and hence higher heat levels. In this respect, car builders can go with larger aluminium overflow or expansion tanks. These perform better in temperature extremes, being more durable, and ensure that there’s no possibility of damage or rupture, so no lost coolant. Aluminium tanks also look much better and fit the whole aesthetic of a reworked engine. Don’t expect to find one for cheap, though.  

Signs of a Bad Overflow Tank 

Bad Overflow Tank
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Like all engine parts, a bad expansion or radiator overflow tank will tell you when it needs replacing. Some more obvious signs are:  

• Overheating – While this is the main symptom of a fault in the cooling system, a tank that is damaged or cracked will leak coolant over other engine parts (and possibly cause corrosion). The lack of coolant in the system causes a quick spike in engine temperature and can lead to melting different parts together.  

• Coolant Smells and Stains – Obvious coloured stains under the car are a sign that the system is leaking coolant. This can also be caused by damage to hoses connecting the different parts. In addition, unusual sweet smells coming into the cabin while driving can point to a coolant leak.  

• Low Coolant Levels – While coolant levels should remain constant, even minor cracks or ill-fitting hoses and connections can mean that small amounts of coolant will be lost to evaporation over time. Periodically check how much fluent remains in the cooling system to avoid the engine from overheating. 

Buying a Tank Replacement 

Overflow and expansion tanks are some of the cheapest parts not only in the cooling system but the engine as a whole. You can go for genuine OEM tanks or aftermarket replacements from a different brand, just ensure that this is compatible with the vehicle. Designs and sizes will vary from vehicle to vehicle (as each engine requires a specified amount of coolant) and tanks need to fit within the allotted space in the engine bay. And lastly, as with everything surrounding cars and engines, shop from respected retailers to know that you’re getting a quality product.