How a Coolant Reservoir Works

The engine under the hood of a red car.

In order to understand the function of a coolant reservoir, it is necessary to first gain an understanding of how the whole cooling system works. You must have noticed that your car’s hood becomes hot after it has been running for some time. In addition, the radiator, set beside the car engine, also heats up. This is because a vehicle’s running engine produces heat when the gas burns. This heat, if not countered appropriately, can cause the engine to overheat and thus damaging it. The cooling system is the only thing that keeps this from happening.

Connection to the Engine

For the purpose of providing a suitable outlet for the heat and keeping the engine cool, vehicles have a cooling system attached to the engine. This system consists of pipes or tubes that pass around the engine, keeping it cool with the liquid that runs through them. A pump pulls the coolant or fluid up and pushes it through the pipes. These run close to the cylinders in the engine block and keep a constant flow of cooling liquid circulating throughout. These pipes connect to a radiator that pulls the heat from the liquid and directs it out of the vehicle.

What Is the Coolant Reservoir?

The coolant reservoir is a container that holds the excess or overflowing cooling agent which is used in the system. It is usually a clear plastic bottle located near the radiator. You can easily find it by following the pipe of the radiator, which will be visible when you open the hood. The coolant reservoir is usually pressurized in newer vehicle models. It is attached to the radiator and the engine with hoses and is a central component in the system. The reservoir itself is the only place where you can actually fill coolant, instead of directly into the radiator.

Coolant to the Radiator

The radiator cap is like the air release system of a pressure cooker. When the radiator is hot, pressure builds up inside it, which releases some of the coolant from the cap. This overflowing coolant passes into the reservoir where it is stored until the engine cools.

When the system chills, the coolant stored in the reservoir is pulled back into the radiator by the reduced pressure. After this liquid passes back into the radiator, the reservoir will normally be left with only 1/3 of the coolant. Therefore, the reservoir works more as a bottle to hold excess fluid until it moves through the pipes.

When Leaks Happen

Any kind of leak or break in the coolant reservoir will make the engine highly prone to overheating. When the vehicle starts and heats up, causing coolant to run into the reservoir, it will simply leak out and be lost. Eventually, no fluid will be passing through the pipes because it will have dripped out onto the ground, and the engine will have nothing working to lower its temperature as it runs.

Unfortunately, since your reservoir is made of plastic, it’s not terribly uncommon for it to develop a crack, especially if you’ve ever gotten into even a small accident. If you have any reason to believe you might have a coolant leak, investigate your reservoir and replace a damaged one immediately.