Coolant Overflow Tank

Coolant Overflow Tank: How Does It Work, And Why It Matters

The car engine consists of several components which generate friction as they rub against each other. The friction raises the temperature in the engine compartment to 195 to 220 degrees Fahrenheit. If the temperature exceeds this level, the engine overheats and will get damaged if left unchecked. Thus, why components such as the coolant overflow tank are important.

The radiator coolant overflow tank stores excess coolant that spills from the radiator. The liquid cools in the tank and returns to the radiator. It is one of the important components in the car’s cooling system that ensures the engine remains at optimum temperatures at all times. 

The coolant circulates around the engine and absorbs the engine’s heat, thereby cooling the car engine. Most car owners may not know how important it is in their vehicles. In this article, we’ll reveal its function and how it works.

How The Radiator Coolant Overflow Tank Works

The car engine burns fuel to produce energy that powers the vehicle. Heat is also a product of that combustion reaction in the engine. The friction between engine parts also constitutes substantial heat that has to be removed, so the engine doesn’t overheat.

The coolant overflow tank is a device in the car cooling system that stores excess overflowing coolant liquid. When the coolant liquid absorbs heat from the engine, it expands, increasing volume. This volume expansion also increases the radiator’s pressure and the cooling system.

The system finds a way to release the excess tension from the radiator. The vapor flows back to the radiator, exchanges heat with the air in the radiator, and cools, returning to liquid form.

The radiator cap is designed to contain a certain pressure. Once it exceeds that level, the coolant passes the radiator cap seal and spills into the overflow tube, and then flows into the coolant overflow tank. The liquid stays there long enough to cool.

When you turn off your car engine, the coolant liquid in the radiator drops in temperature, pressure, and contacts, reducing volume. The volume reduction leaves a vacuum in the radiator, which has to be filled with more coolant. There’s a vacuum valve in the radiator cap; it opens up when there’s a vacuum in the radiator, thereby allowing the coolant liquid in the overflow tank to flow back into the radiator.

Bad Coolant Overflow Tank Symptoms

1. Overheating

Although the engine oil is for lubrication and cooling, it’s not enough to cool the vehicle’s engine. If the coolant liquid leaks away, there won’t be enough of it to cool the engine, and that causes engine overheating.

2. Puddles On Your Driveway

The most visible sign that your overflow tank is leaking is the stain or puddle in your driveway. If it’s damaged, the coolant will leak. This is very common if the overflow tank is old and worn out, so if you’re a car owner, consider changing it before it wears out.

Note: Coolant fluid isn’t the only fluid that leaks from a vehicle or forms a puddle under the car. Engine oil leaks, too, so be sure the stains in your driveway are coolant liquid and not engine oil before seeking solutions.

3. Low Coolant Level

If your coolant levels are unexpectedly lower, it’s evaporating faster than it should or leaking. The leak may not be bad enough to drop on the driveway, so check the radiator coolant level manually to see if it’s a leak and not evaporation.

Note: If the overflow tank cap is not closed correctly, it will cause leaking. Ensure the tank is sealed correctly at all times. Replace the cap if it wears out.

4. Coolant Odor

The coolant liquid is a mixture of ethylene glycol and water, which has a sweet smell. Although the scent isn’t offensive, it’s a bad sign. The coolant liquid is toxic, so you shouldn’t inhale it.

If the overflow tank is leaking, some of the coolant fluid will spread into other areas of the vehicle, even the cabin. If you perceive a sweet smell in your car, the overflow tank is most likely leaking. Check the coolant level in the radiator or look for puddles underneath the vehicle.

Coolant Overflow Tank

Universal Coolant Overflow Tank

You can drive without a coolant reservoir tank. Some cars’ cooling systems use expansion tanks instead of reservoir tanks. If you own such a vehicle, you won’t need a reservoir tank. The expansion cap uses a pressurized cap and relief nozzle, so there may be overflows from the tank at really high pressure. You can install an overflow tank to contain the spills if any. Vehicles without expansion tanks use coolant overflow tanks. The reservoir tank holds the coolant liquid that cools the engine chambers.

Custom Coolant Overflow Tank

You need a custom unit in your car to properly cool the engine compartment. The overflow tank not only stores the overflow from the expanded coolant. The coolant is usually added to the reservoir tank and not the radiator. The coolant flows from the overflow tank to the radiator under pressure.

Small Coolant Overflow Tank

Universal coolant overflow tanks are 1L tanks. To get a small coolant tank, you’ll have to fabricate one. Overflow tanks are designed to hold a certain amount of coolant liquid, if you fabricate a small tank, it may not contain the spills from the radiator, and it’ll leak into other parts of the vehicle.

Aluminum Coolant Overflow Tank

Some tanks are made of aluminum, while clothes are made of polyethylene. Plastic is now more commonly used for reservoir tanks for mass production because it is cheaper compared to aluminum. Polyethylene has high impact strength and moisture resistance making it fit for storing liquids. 

Also, they come as transparent plastics, so you can easily see the coolant level in the tank. However, the strength of the polyethylene fades with time, and the overflow tank wears out. Polyethylene cannot withstand high temperatures, some grades of polyethylene start to melt at 248 degrees Fahrenheit and that causes leaks. You cannot patch plastic coolant tanks. If damaged, you must replace them.

Aluminum tanks offer high thermal capacity and strength, but they cost more. If the tank is damaged, the broken parts can be welded and patched without being replaced. They’re more preferred to plastic coolant tanks. However, OEM automotive manufacturers mostly use plastics to produce coolant tanks because they’re cheaper.

Coolant Expansion Tank Vs Overflow

The coolant overflow tank is a reservoir for coolant liquid. It catches the excess coolant fluid during expansion and stores it. If your vehicle uses an overflow tank, the radiator will have a pressure-rated cap. The pressure from the expanding coolant regulates fluid movement in the cooling system that uses an overflow tank.

During coolant fluid expansion, the pressure in the system exceeds the normal levels the system can contain. The pressure forces the spring in the radiator cap top open, thereby releasing the coolant fluid through the overflowing pot into the overflow tank.

When you turn off the car engine, the radiator cools, and the pressure in the radiator drops. The pressure drop creates a vacuum inside the radiator. The vacuum valve inside the radiator cap pulls back the coolant liquid from the overflow tank back into the radiator to equalize the pressure.

The expansion tank is very different from the overflow tank and is more complex. The radiator cap is attached to the expansion tank because it is always under pressure. If the pressure exceeds the tank capacity, the cap opens to release the coolant. The radiator may not have a cap or fill neck since the coolant is poured directly into the tank.

As the coolant expands and increases the pressure in the cooling system, the pressure fills the space above the expansion tank. This pressure inside the tank forces the coolant back into the cooling system. The coolant never spills into a reservoir to cool; it is constantly cycled from the radiator to the expansion tank.

DIY Coolant Overflow Tank

You can design a simple overflow tank by yourself if you have an understanding of how the reservoir works and the right materials. The overflow tank in most automobiles is made of plastic materials and some with aluminum. You can design your own overflow tank from scratch with a plastic bottle.

You’ll need:

  • Nylon Tight-Seal Barbed Tube Fitting.
  • 3 90 Degree Elbow.
  • 8oz plastic bottle.
  • Hose.
  • High-Temperature Silicone Rubber Tubing.


  • Drill two holes in the cover of the plastic bottle.
  • Rub epoxy glue on the two 90-degree elbows and fit them into the holes.
  • Apply more epoxy to the joint between the fitting and the bottle cover.
  • Trim the barb pipe and clean the edge with rubbing alcohol.
  • Apply epoxy glue on the barb pipe and push it into one of the 90 degrees elbows from the inside of the bottle cover.
  • Put more of the glue on the joints between the elbow and the trimmed bab pipe to strengthen the hold.
  • Apply epoxy glue on a hosepipe and stick it with the bab pipe on the inside of the bottle cove. You’ll immerse the hose pipe into the coolant liquid in the DIY overflow tank.
  • Attach the silicone tubing to the 90 degrees elbow on the bottle cover.

Note: The hose immersed in the overflow tank will receive overflow coolant into the tank and send it to the radiator. The process is regulated by pressure. The silicone tubing will act as the inlet and outlet ports on the standard overflow tank.

Oil In Coolant Overflow Tank

If you just opened your car’s coolant overflow tank and noticed there’s oil in it, the possible cause is that the oil/coolant heat exchanger or the head gasket is broken. Or maybe you poured the wrong fluid into it the last time you were topping the coolant level.

The oil-coolant mixture is a big issue and should be fixed as quickly as possible, so it doesn’t get into the radiator. These are not the only possible ways oil can get into it. Here are more details on why there’s oil in the overflow tank.

1. Faulty Oil Cooler

If you’re driving a modem model vehicle, your car probably has an oil cooler installed; the coolant liquid cools the oil cooler. A broken oil cooler is a common cause of oil mixing with coolant. The oil cooler wears out with time, and its oil content leaks into and mixes with the coolant. The good thing is, if the oil cooler is the fault, you won’t spend as much money as you would if it’s a broken head gasket. Oil cooler replacement costs between $525 and $580.

2. Broken Head Gasket

If there’s oil in your car overflow tank, a damaged head gasket is the most likely cause. The head gasket is found in between the engine block and the cylinder head. It’s a rubber seal, so its role is to prevent oil leaks from the engine. It also seals the combustion mixture of air and fuel from the engine chambers from escaping so that it can build compression.

The engine oil will leak into the cooling system or coolant if the head gasket is blown. The head gasket is quite expensive. It costs $2,000 to $4,000 to replace the head gasket. Do your part to preserve the lifespan of your vehicle’s head gasket by maintaining the correct levels of the coolant always.

3. Cracked Cylinder Head

An overheated engine not only damages the head gasket but cylinder heads too. The material they’re made of cannot withstand extreme temperatures, so they crack in some places. Oil leaks through those cracks and into the coolant, although this rarely happens. Depending on which point in the cylinder head cracks, you can weld the cracked part or get a new one. The cost of replacing the cylinder head is between $3,366 and $3,705.

4. Cracks In The Engine Oil

A cracked engine is a result of overheating. The engine oil that should be oiling the system will leak, and some of it gets into the coolant. The downside to this is that the car engine cannot be welded. If it is damaged, it has to be replaced. Car engines cost about $4000 and above, depending on the vehicle type and labor cost. As a preventive measure, maintain correct oil and coolant levels in the vehicle at all times.

5. You Poured Oil Into The Coolant Overflow Tank

An oversight can make you pour oil instead of coolant fluid into the overflow tank. Cross check that you have the right container each time you add coolant to the overflow tank.

Coolant Is Leaking From Your Overflow Tank

1. Coolant Level

If too much coolant liquid is in the radiator, too much of it will spill into the coolant overflow tank. After absorbing heat from the engine, the coolant liquid expands and flows to the overflow tank to reduce pressure build-up. With an excess of the coolant fluid in the radiator, more of the coolant will spill into the overflow tank. Once it exceeds the volume of the overflow tank, some of the liquid will come out of the tank.

2. Faulty Water Pump

The water pump regulates the car cooling system. It pushes the coolant liquid through the engine and back to the radiator. If the water pump is faulty, it won’t circulate the coolant properly. Too much coolant liquid can escape into the overflow tank and leak.

3. Radiator Cap

A bad radiator cap is a common cause of leaking overflow tanks. The radiator tank is designed to contain the liquid in a closed system. Excess coolant goes into the cooling system if the cap is damaged, and subsequently, the overflow tank leaks.

4. Thermostat

A faulty thermostat can also cause leakage in the overflow tank. The thermostat is a temperature-sensitive device that opens when a temperature rises. When there’s excess heat in the engine, the thermostat opens up, signifying that the engine needs to be cooled. The water pump then pumps coolant liquid into the radiator and engine. Coolant liquid will keep circulating if the thermostat is stuck in the open position until the overflow tank gets filled and leaks.

5. Radiator

Sometimes, an overflow tank leak is caused by a faulty radiator. Modern vehicle radiators have a lot of plastic components; they wear out and cause a release of excess coolant into the system. If the radiator leaks, hire a professional to replace it.

Coolant Overflow Tank

Coolant Overflow Tank Full But Radiator Empty

If you’re doing routine checks and find your car radiator empty, but the tank is full, it’s a sign of damage to the cooling system. The coolant expands and flows into the overflow tank but doesn’t return to the radiator after it cools.

Under normal conditions, the vacuum valve in the radiator cap draws back the coolant into the radiator. If the overflow tank is full, but the radiator is empty, the overflow tank is leaking, or the hose is clogged. Another possible reason is a damaged radiator cap. If the radiator cap is broken, it will release more coolant from the radiator than it should, causing it to go empty.

If you notice the radiator is empty, fill the radiator with coolant to prevent engine overheating. Take the vehicle for proper diagnosis by an expert to know where the fault is and repair the car accordingly.

Facts about Car Radiator Coolant Overflow Tanks

  1. Without additional equipment, the coolant system in a car can be damaged by rust due to how coolant works in a radiator.
  2. Rust can form in the cooling system of a car due to air getting into the radiator after the engine has cooled down.
  3. When the coolant cools after the engine has been turned off, it contracts in volume, causing a drop in temperature and pressure inside the radiator, allowing air to enter the radiator through the radiator cap.
  4. The air entering the radiator results in the formation of rust in the cooling system when it comes into contact with the metal in the system.
  5. A radiator coolant overflow tank can prevent rust formation in the cooling system, saving the expense of frequent coolant replacements and potentially expensive repairs or replacement of cooling system components.
  6. The radiator overflow tank prevents rust formation by allowing excess coolant to flow into the overflow tank when the pressure in the radiator surpasses the designed pressure of the radiator cap.
  7. As the coolant cools down, it contracts in volume, resulting in a vacuum because of a drop in pressure.
  8. Due to this vacuum, a vacuum valve in the radiator cap opens, allowing the coolant that flowed into the radiator coolant overflow tank back into the radiator.
  9. Older vehicles without coolant overflow bottles can suck air into the radiator instead of coolant, resulting in rust formation.
  10. Installing an overflow tank in a car can maintain the cooling system and save money on repairs and replacement expenses in the long run.

Frequently Asked Questions

Here are some popular FAQs:

How Much Coolant Should Be In The Overflow Tank

Under normal conditions, the coolant overflow tank should not hold fluid above 30% of its volume. On the sides of the overflow tank, you’ll see the full and low marks denoted by F and L. When pouring coolant liquid into the overflow tank, make sure it’s slightly above the minimum. If the coolant in the overflow tank passes the maximum mark, it’ll leak into other parts of the car.

Why Is Coolant Coming Out Of The Overflow Tank

A common reason coolant liquid comes out of the overflow tank is that there’s too much coolant liquid in the vehicle. A bad radiator cap or faulty water pump causes coolant tank overflow.

What Does It Mean When The Coolant In The Overflow Tank Is Boiling

If the coolant in the overflow tank is boiling or bubbling, it indicates a cracked or blocked head gasket which has to be replaced. A cracked head lets air into the cooling system, thereby increasing the air pressure. The air forms bubbles in the overflow tank. Take the vehicle for head gasket repair or replacement if you notice this boiling. Note: Never open the radiator cap while the engine is still hot. At high temperatures, there’s pressure build-up in the radiator. If you open the radiator cap in this condition, the coolant liquid will spray out and burn or scald you.

Why Is My Coolant Overflow Tank Full

The coolant overflow tank gets filled up when there’s an excess of coolant in it. The coolant reservoir should never hold coolant of more than 30% of its volume. If the overflow tank gets filled up, it’ll leak eventually. If your vehicle overflow tank is full, identify the cause of a faulty thermostat, radiator cap, or water pump.

Should There Be Coolant In The Overflow Tank

Yes, it is normal for some coolant in the overflow tank. The primary purpose of the overflow tank is to collect excess coolant that drains out of the radiator when the coolant liquid expands. Without the overflow tank, the spills will flow into other vehicle parts. When the coolant levels in the vehicle drop, you’ll top it by adding coolant into the overflow tank.


The coolant overflow tank in vehicles is an integral part of the car cooling system. It stores overflow from the radiator, drawn back into the radiator after the engine cools. The coolant tanks can get damaged with time, especially the plastic ones with less thermal strength. If you notice leaks, check if the overflow tank is damaged and repair it accordingly.

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