That odd plastic tank next to the radiator isn’t just for decoration – it has a purpose, and it’s called the coolant overflow tank. When engine heat causes the coolant to expand, where does all of that extra fluid go?
The overflow tank takes over and makes sure the coolant doesn’t overflow and cause a mess. It’s an incredible relief valve that functions when the system expands to help manage the temperature of your engine while driving your car.
Let’s take a closer look at the inner workings of your coolant overflow tank.
How a Radiator Overflow Tank Works?
The radiator overflow tank, also known as a coolant reservoir or expansion tank, is a simple device that has two purposes:
- To capture the coolant that is expanding and would go on the ground.
- To hold that coolant until it’s needed back in the radiator. The two work together to form a closed system that keeps your engine’s coolant at the correct level.
As your engine warms up and the coolant heats up, it begins to expand. The coolant being trapped in the cooling system expands into more volume, creating pressure.
When the pressure exceeds the amount the radiator’s cap can handle (usually 12 – 15 PSI), the coolant blasts past the cap’s seal and out through the overflow tube into the overflow tank.
Once you turn off your engine and the temperature begins to drop, the coolant shrinks and leaves less volume of liquid in its place. This creates a vacuum effect in the radiator as the pressure drops.
The vacuum opens a valve on the radiator cap, pulling coolant from the overflow tank back into the radiator when the engine cools enough.
Advantages Over Older Systems
The system has a huge advantage over older systems with no overflow tank. With an overflow tank, there is no possible waste of coolant, it can maintain correct pressure from the cooling system, and it assists with removing air bubbles from the cooling system.
Older systems with no overflow tank would pull in air and not coolant when the engine cools down.
Here is how it all ties back together… The introduction of air into the cooling system could result in rust formation. The overflow tank provides the cooling system with a proper fill and keeps it protected.
Design
Most overflow tanks are constructed from see-through/somewhat see-through plastic. The see-through lets you check the coolant level without opening the system.
In general, an overflow tank will have “MIN” and “MAX” markings on the side, and you should see the coolant level fluctuate within these markings when the engine is cold.
If you see the level drifting downward or you have a lot of bubbly coolant in the tank when the engine is running, your cooling system likely needs attention.
System Components
There are three main components to an overflow tank system that must work together properly. The first component is the radiator cap. This component may look simple in form overall, however, it is a precisely engineered component with two separate spring-loaded valves that work together to maintain the system’s integrity.
The pressure relief valve on the radiator cap opens when the system has exceeded its pressure limit. So, coolant from the radiator can flow into the overflow tank. The vacuum valve opens when the system is cooling, and it allows coolant to return to the radiator.
The overflow tank itself is usually mounted higher than the radiator. This is done to help ensure that there is a flow of fluid freely.
All overflow tanks are constructed from plastic, except for high-performance vehicles. High-performance vehicles will typically use metal tanks to provide more durability.
The tank must be properly mounted because cracks from vibration are one of the most common failure points.
Most overflow tanks will have a short rubber hose connecting to the radiator’s filler neck, this hose connects the radiator’s filler neck to the overflow tank. This rubber hose must remain clear and properly connected to make the system work.
Any blockage, crumbling, or disconnection of the overflow hose can mean that coolant cannot freely flow passage to/from the radiator to/from the tank, and overheating or coolant loss could occur.
For many vehicles, the overflow hose connects to a small pipe or nipple on the neck of the radiator. Sometimes this nipple can corrode, and if this occurs, frequent cleaning or replacement may be necessary.
Types of Coolant Expansion Tanks
Most vehicles use either a pressurized tank or a non-pressurized tank for the overflow.
The non-pressurized tank is a simple reservoir that connects to the radiator with a single overflow hose. The tanks are simple plastic containers that are connected to the radiator. The cap is vented, allowing air to escape.
The pressurized tank, which is also called an expansion tank or degas bottle, is part of the sealed cooling system. It maintains system pressure. It has a pressure cap similar to a radiator cap, two hose connections, and is part of the sealed system.
Most of the pressurized tanks are prevalent in many of the newer domestic models, but are also common in European models. The pressurized tank has become a replacement for the radiator as the primary fill-up point for coolant.
Most coolant overflow tanks are made from plastics, but some higher-performance or heavy-duty vehicles may use metal coolant tanks for added durability. Most consumer automobiles will utilize plastic tanks and these are typically marked for the coolant amount to fill.
Maintenance and Inspection
Physical Condition
First, check the tank’s physical condition. Check the tank for cracks, discoloration, or deformation, concentrating around the mounting points and where the hoses connect.
These tanks can stiffen over time due to heat exposure and engine vibration, so any leaks, damage, etc., usually indicate the tank should be replaced.
Coolant Level
Make it a habit of comparing the coolant level to the MIN/MAX marks on the tank when the engine is cold. If you see the level keeps dropping, you might have a system leak.
However, having an amount above MAX is not good either, and may lead to excessive coolant being forced out through the cap.
Hose Connection
Check the overflow hose for signs of cracking, softness, or deterioration. Ensure the hose is secured connecting to the tank and that it is not blocked or kinked. A loose or damaged hose will not allow the system to work as it should.
Coolant Condition
When you visually assess the coolant in the tank, it should be clean and free of debris. If you see any floating particles, oil, or excessive rust colored coolant, you need to flush the cooling system. If the coolant is a milky color, it often points to a major concern like a head gasket leak.
Cap Inspection
For pressurized tanks, examine the cap seals; ensure the seals are in good condition (no cracks or hardened plastic). If the cap cannot maintain the proper system pressure, you may have some IWE issues later on.
If you have a non-pressurized tank, it is important that it can vent properly but still keep any debris from entering the tank.
Can a Radiator Pull Coolant From the Overflow Tank?
Yes, a radiator can pull coolant from the overflow tank. In a cooling system with a radiator cap and a separate overflow tank, coolant can flow in both directions.
As the hot engine causes the coolant in the radiator to expand, it pushes the coolant into the overflow tank via the overflow tube.
Once the engine cools and the coolant level in the radiator drops, the radiator pulls liquid back in from the overflow tank. This ensures the coolant isn’t lost while also allowing for coolant expansion and contraction.
The overflow tank is also intended to hold air in the system, but not in the same manner as just explained. While air bubbles can collect in the overflow tank, they do not separate from the coolant automatically when returned to the radiator.
In essence, the overflow tank’s air-only function would be primarily to allow air to expand and not allow the air to build pressure enough to push the coolant out of the system.
Ultimately, the overflow tank functions primarily as a reservoir for coolant volume and pressure excesses.