How Does a Radiator Overflow Tank Work?

A radiator overflow tank is a reservoir for excess engine coolant that would rise up and out of the radiator due to the pressure built up after reaching the boiling point. It is a relatively simple device that can save the cooling system of your car in many ways. It works much like the expansion tank on a water heater.

Significance Of the Radiator Overflow Tank

Engines that work hard heat up quickly. It is a common observation. The components of the engine need to be cooled, but just oil is not enough for it. So, for this type of high-performing situation, the engine depends on a coolant liquid. It avoids overheating and keeps the engine cool by sucking up all the generated heat.

Apparently, the cooling system of the car can work fine without any additional tool or device, but there was a problem. In traditionally cooled vehicles, the excess coolant was expelled into the atmosphere or dumped onto the ground. The antifreeze component of the coolant mixture is toxic. So, it is not only hazardous for the atmosphere but also health.

Moreover, rust can also be formed inside the cooling system. It is because of the air that can enter into the radiator through the radiator cap after the engine cools down. Upon cooling, the volume decreases, and in consequence, temperature and pressure also drop. It allows the air to move into the radiator and form rust after contact with the system’s metal.

So in 1970, environmental awareness added the overflow tanks to the system to trap the emitted fluid and prevent rusting. It keeps your radiator functional for a long time and saves you from frequent coolant replacements that are irritating and expensive.

How Does a Radiator Overflow Tank Work?

Every engine performing at its optimum needs a coolant that can keep it within the operational temperature. The engine coolant works by absorbing the engine’s heat, and upon heating in the radiator, it starts expanding. An increase in volume automatically increases the pressure in the cooling system of the vehicle.

Then, a point comes where the pressure causes the coolant to get higher than the radiator cap’s designed pressure. The antifreeze passes the radiator cap seal and prevents leakage; it has to be captured somewhere.  And here, the radiator overflow tank serves the function. The coolant flows into this reservoir through the overflow tube.

Once the engine is turned off, the heat starts dissipating, and as a result, the coolant also begins to cool down. Now recall your 9thgrade chemistry, where you have learned that a decrease in temperature decreases the volume. So, the fluid also contracts here.

Moreover, the decrease in volume results in a pressure drop. It creates a vacuum in the radiator. This vacuum effect opens a vacuum valve in the radiator cap that permits the coolant to move back into the radiator previously fled into the radiator overflow tank.

The old vehicles that lacked radiator coolant overflow tanks were more likely to form rust and cause environmental pollution. It is because, in those systems, the radiator sucks air instead of the coolant. And it results in the formation of rust. So, the installation of radiator overflow tanks in the vehicles is a major achievement. They will maintain the cooling system of your car and save you from expensive repairs and frequent replacements.

Bad Overflow Tank Symptoms

Now you are very well aware of the importance of radiator overflow tanks in vehicles. So imagine that what would happen if your tank is faulty, cracked, or overflowing. Here comes the question that how would you know that your overflow tank is out of order. Here are the most common symptoms that tell you that your radiator overflow tank needs repairing or replacement.

Coolant Leak

If you notice small drops and puddles of coolant on your garage surface or your driveway, it is definitely your radiator overflowing tank that is leaking. Coolant fluid leaks out of the tank if it is cracked or damaged.

If your reservoir is too old and worn out, you should consider replacing it because the leaking fluid is not good at all.  It is dangerous for dogs and cats. So, if you notice some coolant on your floor, do immediate cleaning to save your pets from getting sick just because of licking the engine coolant.

Overheated Engine

If your engine gets overheated, the coolant is not doing its job since its job is to keep the engine cool during optimum performance. And if it is not doing so, it means that the engine is not getting enough coolant to cool it down.  Here is an indication that you should check your radiator overflow tank because its malfunctioning may cause the problem.

Coolant Odor

You can feel a specific smell of the odour in the front of the vehicle, but if it gets bad and strong, it can be a warning sign for you to check your car’s condition, especially its radiator and overflow tank.

Although the smell is not the worst one, nobody wants to drive with it for very long.

Low Level Of Coolant

If you have recently filled your coolant tank and the level is down after some time, there might be a small leak or slow evaporation. These leaks are barely noticeable, so to make sure, you have to check the coolant level. Mostly, the overflow tank cap causes the leak, so close it correctly or replace it if you still suspect the fluid escaping through it.


In this article, you have learned about how a radiator overflow tank works? It is a simple yet effective process that keeps our environment safe from pollution. It also increases the radiator’s life and keeps you away from the expenses of replacing it over and over again.

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