What Causes Coolant Reservoir To Boil? Causes & Solution

Does your coolant reservoir boil? Your cooling system has trouble. If your coolant isn’t appropriately circulated, it will overheat, and your engine will seize up. Afterward, it can significantly damage your engine. Boiling coolant or water can also crack the radiator, causing leaks.

If the water boils in your reservoir tank, your engine will overheat. Finally, raising temperatures adds more pressure to the cooling system if the head gasket goes.

It will be difficult for you if you are trying to figure out what to do. That’s why you should know what causes the coolant reservoir to boil. Its answer depends on the severity of the problem. However, I don’t think so because here we will explain the reasons and solutions to this problem. So, please stay with us.

Coolant Reservoir Is Boiling

What Does It Mean When Your Coolant Reservoir is Boiling?

If your coolant reservoir is boiling, your car is probably overheating. Boiling coolant means the coolant level has dropped too low or the water pump is faulty. Both are responsible for overheating your vehicle.

Boiling coolant can’t dissipate heat properly, which can lead to engine damage over time. To solve the problem, you should find the reason and repair it before severe engine damage occurs.

You should check the water pump and coolant level as a first step. If your coolant level is too low, you need to top up your coolant. But if your coolant pump is a problem, you should repair it.

Main Causes Of Coolant Reservoir To Boil:

There are several possible causes of your coolant reservoir boiling. And it’s essential to find out the problem as soon as possible. Here are some reasons why coolant reservoirs boil.

Engine Overheat:

The Main culprit for the Coolant or water boiling is the faulty engine which may produce overheating for the different mechanical problems.

Several possibilities need to be considered. Having the gauge idle to indicate when the temperature will actually rise. If the engine warms up after starting driving, you should look for airflow problems. For example, the airflow problem will be blocked or rotten radiator fins and fan clutch.

A missing radiator can also cause the engine to overheat. Water flow problems will occur at highway speeds. It can be the reason for a clogged water tube, thermostat, or water pump.

So if your engine continuously produces excessive heat then your coolant reservoir may start to boil.

Faulty Thermostat

A faulty thermostat is also the reason for a boiling coolant reservoir. If the thermostat is stuck in the “open” position, the coolant will flow through the engine too quickly, causing it to overheat.

Additionally, if the thermostat is in the “off” position, it can obstacle the coolant from circulating properly, leading to overheating.

Bad Cooling Fan

Excess heat is generated during air compression when the vehicle is running, and mechanical parts friction needs to be dissipated. The car’s radiator fan has a coordinating role with the coolant to increase the cooling performance of the engine.

For the blown fuse and failed fan clutch, if your cooling fan destroys, it will start boiling your coolant. It’s because the fan is not providing enough air circulation when your car is stopped. Regardless of the cooling fan setup, its primary purpose is to cool the coolant. If it fails, you will see your coolant boiling.

Failed Radiator Cap

The radiator cap is usually neglected for maintenance, but it’s an essential part of the cooling system. A radiator cap means a release valve. If your coolant reaches a high temperature, the release valve opens, ensuring that your cooling system is under pressure.

But if your cap is clogged, it can’t maintain enough pressure to lower the coolant boiling point. As a result, the coolant absorbs the insufficient heat from your engine.

A coolant air pocket can overheat your engine because the radiator cap does not work correctly, and the air increases rapidly. At last, it can lead to coolant in the reservoir boiling.

Clogged Radiator

Adequately, if your coolant starts to boil, check the radiator. Like other vehicle parts, radiators can also be affected by wear.

Eventually, debris can enter the radiator and start to clog it. Remember that the radiator ensures you the cooling system works effectively. So if your radiator gets clogged, your engine will overheat. Resulting in your coolant temperature will raise abnormally.

Low Coolant Level

Riding your car with low coolant will create airlocks and foamy fluid in your cooling system. Airlocks will disrupt the cooling system flowing, causing the engine to overheat.

You have a low coolant level if the warning light is on your dashboard. If you don’t notice, the engine will start boiling.

Check: How Much Coolant Loss Is Normal?

Incorrect Coolant Mix:

The cause of your coolant reservoir boiling is the wrong coolant mixture. If the water and antifreeze ratio is off, your coolant will overheat and boil.

When topping off your fluid, remember to use the proper water-antifreeze ratio. Using the correct antifreeze for your vehicle is also vital. Please check your owner’s manual or ask a mechanic for more clarification.

Check: How Much Coolant Do I Need For My Car?

How To Fix Coolant Reservoir To Boil?

As mentioned in the section above, there are many reasons for your coolant boiling in the reservoir. So if you don’t identify the exact cause, it can cause severe consequences to the engine.

Therefore, you should go to the mechanic to know the reason and fix it immediately after discovering the cause. However, here we give some solutions to how to fix antifreeze boiling in a reservoir. Check it out:

Replace Bad Thermostat

If the thermostat is clogged, it obstructs coolant from circulating the engine to the radiator. In this case, replacing the thermostat is a good idea. Fixing a bad thermostat problem can be simple for most DIY enthusiasts, but only for some.

If you can repair your car yourself, you can replace the thermostat. If you don’t, you should take your vehicle to a mechanic.

Fix An Air Pocket in The Reservoir

You can easily manage the problem by adequately filling it with coolant. Yet, when changing the coolant, used air is clogged inside the tank.

As a result, the air bubbles and temperature of the coolant rise. Therefore, to ensure no air is in your reservoir, close the radiator cap and start your engine. It can help to remove the air from your tank.

Replace Bad Radiator Or Radiator Cap.

You should check your radiator cap to fix the car radiator water boiling. Typically, if your pressure drops, you should clean this cap and try again. Moreover, if you can get the same result, replace your radiator cap now.

Replacing the cap can be easy; the radiator cap is generally cheap. Though you can buy a new one, you should ensure the proper pressure rating.

After maintaining it, your radiator will last up to 8 to 10 years. In addition, when you notice coolant boiling, replace your car radiator.

What Causes Coolant Reservoir To Boil

FAQ (Frequently Asked Questions)

Q. How do I stop my coolant from boiling?

To stop your coolant from boiling, you should do the following steps:

  • You should stop the engine when you notice the temperature gauge is red.
  • Please do not touch or try to remove the coolant cap; you may burn yourself badly.
  • Let the vehicle sit for 30 minutes before removing the coolant cap.
  • Find reasons like sticking thermostats, leaky coolant hoses, leaking radiators, etc.
  • Check to ensure the correct amount of fluid inside the coolant overflow tank.

Q. Why is my coolant reservoir boiling, but the engine doesn’t overheat?

If your coolant reservoir is boiling, but your engine isn’t overheating, your coolant may not actually be overheating. The bubbles you notice are likely air bubbles — not from the piping-hot coolant. In most cases, the problem is even a leak in the radiator cap.

A water pump, head gasket, or overflow tank leak can also cause bubbles in your coolant reservoir. The coolant system is highly pressurized and tightly sealed. So any leak can obstacle the coolant from working at its best.

Since there are many reasons for bubbles in your coolant, we recommend you take your car to a mechanic. Time is valuable: If you take your vehicle to a mechanic late, you will need expensive repairs.

Q. Can a bad thermostat cause bubbling in the coolant reservoir?

A faulty car thermostat controls the radiator’s coolant flow in and out. In this case, the thermostat should be opened and closed at specific times. It would help if you opened it to let the hot coolant out of the engine and cool in the radiator.

Stay close to the car because the engine heats up to start the coolant. When a thermostat malfunctions, it can’t open and close at the correct time. As a result of uncontrolled airflow, it creates a bubble in the coolant reservoir or radiator.

Final Word

Most drivers want to know what causes the coolant reservoir to boil. So if you face this situation while driving, you should stop the car immediately to avoid further damage.

Hopefully, the information we shared about the reasons and solutions will help you to drive safely. Thanks for reading this article attentively.


I am Engr. Z Eusuf Lubricant Specialist, Mechanical Engineer & has been working with engine lubricants for a long time and knows what makes them work best. I can help you find the right oil for your needs, and I have tips on how to keep your engine running at its best.