6 Symptoms of a Bad Radiator Coolant Overflow Tank (and Replacement Cost)

The purpose of the radiator coolant overflow tank is to store excess coolant that is released from the radiator due to pressure build up from heat. It works similar to an expansion tank on a water heater.

As you may know, an engine that is working very hard will heat up very quickly. The oil alone is not enough to keep the components of the engine cooled off.

The engine relies on coolant to ensure that it does not overheat during these high performing situations. While this heat is being generated, the coolant liquid will absorb all that heat so that the engine stays cool.

So what would happen if the coolant overflow reservoir tank became cracked, damaged, or overflowing? Below are common symptoms that may appear with a faulty coolant expansion tank.

That small plastic tank next to your radiator probably seems insignificant, but it plays a critical role in keeping your engine running safely. When the radiator coolant overflow tank starts to fail, it can set off a cascade of problems, from mysterious leaks to catastrophic overheating issues.

Learn how to recognize the warning signs of a compromised expansion tank before you get stranded with an overheated engine. The good news is that replacing the coolant overflow tank isn’t as expensive as it could be.

Common Signs of Coolant Expansion Tank Failure

Here are six of the most common signs of a bad radiator overflow tank.

1) Coolant Leak

coolant leaking

If the radiator coolant overflow tank is damaged or cracked, then you can expect coolant fluid to leak out of it. Sometimes cracks will form on the overflow tank if it is too old and worn out. You should notice small puddles or drops of coolant on your garage floor or in your driveway.

Note: Coolant is very harmful to dogs and cats. If you notice a coolant leak on the ground, get it cleaned up right away so you family pet doesn’t clean it up for you. There have been many cases of dogs and cats getting very sick or even dying because they ingested engine coolant. 

2) Overflow Tank Cracks or Cloudiness

The plastic used to make radiator overflow tanks can become brittle and prone to cracking over time. Even small hairline cracks can lead to leaks of vital engine coolant. Visible cracks in the tank, especially near mounting points, indicate it needs to be replaced.

Cloudiness or staining inside the tank points to corrosion and breakdown of the plastic due to age or chemical reactions. As plastic degrades, it allows small leaks even without visible cracks. A cloudy overflow tank also suggests contaminated coolant that requires a flush and refill.

Checking for cracks or cloudiness when the engine is cold takes just a minute and can easily prevent expensive engine damage.

3) Coolant Odor

car smells

In addition to the coolant leak, there will be an apparent coolant smell coming from the front of your vehicle. If it gets bad enough then it may even circulate throughout the cabin of your vehicle.

While not the worst smell in the world (it’s slightly sweet), it’s not something you want to smell for long periods. So if you smell this warning sign, check the condition of the overflow tank and replace asap.

4) Overheated Engine

Since the coolant’s job is to cool the engine when it is overworked, a leak (or a collapsed radiator hose) may mean that your engine is not getting enough coolant circulating through the engine to adequately cool it down.

As a result, you can expect the temperature of your engine to rise and become overheated. This will be indicated on the engine temperature gauge located on the dashboard.

See Also: 8 Bad Coolant Temperature Sensor Symptoms

5) Low Level of Coolant

coolant leak

If you check your coolant levels in the radiator coolant overflow tank and they are low, then you either have slowly evaporating or leaking coolant. Usually, people who don’t realize they have a leak may discover that it is a small leak which is barely noticeable on the outside.

To confirm, you need to actually check the coolant level to make sure you have a leak. The overflow tank cap is often the cause of the leak. Make sure it is correctly closed and replace the cap if that’s where you suspect coolant is escaping from.

6) Bubbles in the Overflow Tank

Normally, bubbles in the coolant reservoir indicate problems with the cooling system. Air bubbles signify that combustion gases from the engine cylinders are entering the coolant, often through a failing cylinder head gasket.

Bubbles can also result from cracks in engine components like the cylinder head, block, or water pump housing. These allow the pressurized mix of coolant and gases to leak out.

While less likely, bubbles in the coolant expansion tank may simply indicate there is a crack or hole somewhere in the tank which is allowing air in.

Radiator Overflow Tank Replacement Cost

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coolant overflow reservoir replacement cost

Before spending the money on a new overflow tank, make sure the problem is not actually the overflow cap or tubing that comes out of the tank at the bottom as these are common problem areas. They are a lot cheaper to fix than replacing the tank itself.

Otherwise, the cost to replace a radiator coolant overflow tank is anywhere from $90 to $260. The parts cost will be between $50 and $150 while the labor costs will be anywhere from $40 to $110.

On many vehicles, replacing the overflow tank is an easy DIY job. On others which are harder to get to, you should let the professional handle the replacement job. There will also be additional fees and taxes added to this as well.

How a Radiator Overflow Tank Works

As coolant gets hot due to it absorbing heat from the engine, the liquid expands and creates additional pressure in the radiator. As the pressure causes the coolant to get higher than the radiator pressure cap, the excess coolant needs to be captured somewhere in order to prevent leakage.

So, the radiator coolant overflow tank functions to serve this exact purpose. The excess fluid flows into the overflow tube and goes into the overflow tank. Once the driver parks the vehicle and turns off the engine, the heat dissipates which causes the coolant to not be as hot anymore.

The coolant will then contract instead of expand; resulting in the volume of the coolant decreasing. This is kind of like a vacuum effect where the pressure decrease allows the excess coolant in the overflow tank to flow back out of it, so it can return to the radiator.

Causes of Tank Failure

Your radiator coolant overflow tank may fail due to several reasons. Identifying the root cause is the first step of repair and making sure the replacement tank doesn’t have the same outcome.

Age and Wear

As your vehicle ages, so do its parts, especially those made of plastic. Your overflow tank is not immune. Years of enduring extreme temperature fluctuations can degrade the tank’s material, leading to cracks and leaks.

Excessive Pressure

When the radiator cap fails, it can’t maintain the right pressure, causing coolant to escape into the overflow tank. This can result in overflow or even damage to the tank due to the pressure beyond its design limits.

Chemical Damage

Coolant contains chemicals that over time can corrode the tank from inside out. Proper mixture and regular changes can prevent the chemical wear that leads to tank failure.

Physical Damage

External factors like accidents or debris can inflict physical damage since coolant reservoirs are typically located along the perimeter of the engine bay. This is good since it means more expensive interior engine components are better protected but bad since the tank can often take an almost direct hit.

Even minor impacts can compromise the tank’s integrity, so it’s important to inspect for any signs of physical harm.


15 thoughts on “6 Symptoms of a Bad Radiator Coolant Overflow Tank (and Replacement Cost)”

  1. My textron overflow coolant tank has a small crack in it. So it shoots out when its running. My question is can one take a two part epoxy glue and stop the leak?

  2. I have the same issue. Coolant in the reservoir does not to go back into the radiator after I turn off the engine. I have replaced the radiator cap twice with no changes

    • Is the coolant overflow reservroir overflowing because the coolant will not go back into the radiator? When you check the radiator cold, how is the level?

    • Your thermostat gauge may be broken, or the thermostat may be stuck open. It might be a good idea to have someone take a look at it. Engines aren’t as efficient if they run cold all the time, and you wouldn’t want a broken thermostat to suddenly get stuck in the closed position. That would likely overheat the vehicle.

  3. I have similar problem and all my mechanic seems not to have perfect solutions, i already change to new radiator but it continues and i was advised to Change the top gasket, please @chijioke. Which city are you based i need your help to fix mine, am frustrated by this

  4. I had the same problem. But i did the following by myself and the problem was solved.
    1. Disconected the return water hose from the engine and drained the radiator completely.
    2. Opened the radiator cap.
    3. Removed the overflow tank cap and blow air (with my mouth) through the cap hose .
    4.while leaving the radiator return hose disconnected, continue filling the raditor wlth water while the engine still on. This helped me flushed any debris that may have accumulated in the system.
    5.Removed the overflow tank and cleaned properly.
    6. Fixed back the radiator cap,hose ,overflow tank and cap and filled the radiator completely with water.
    7. Started the car and noticed that the problem has been solved after driven the car for 2 days.
    8. Everything is now good. No more problem.

  5. I have the same problem with my 2008 PT Cruiser. Coolant burps into the overflow bottle, but then will not return to the radiator. What’s up? There is still pressure on the cap 12 hrs after I turn the car off.

  6. Good article. I have replaced my overflow tank, radiator and cap, checked my oil and no coolant, in my oil.. my dealership say the thermostat is good. When on a trip 200 miles my coolant goes into the overflow tank but it does not return to the radiator. I have to manually pump the fluid back into the radiator. Why do I not have the vacuum to suck the fluid back to the radiator? 2013 CHYSLER 200.


    • My guess is that, radiator has a pressure cap so in order to actuate it it needs a pressure. But there is no way to return the coolant from overflow tank as the cap needs pressure and in reverse direction.


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