Why Is My Coolant Reservoir Boiling? – 6 Top Causes

Why Is My Coolant Reservoir Boiling? – 6 Top Causes

If you do not know why your coolant reservoir is boiling, it can be challenging to sort out the problem right away.

It may be due to a failed thermostat, a blown head gasket, or even rust.

Why Is My Car Coolant Reservoir Boiling

Before resolving the issue, you will have to diagnose the cause behind your car coolant reservoir boiling. Here are some reasons why this may be happening, explained:

Air Pockets

Sometimes, when you refill your coolant, you may follow an improper flushing procedure which can cause air pockets to be trapped in the cooling system.

When the engine is running and heating up, these air pockets may expand and move into the coolant reservoir.

When the engine cools down again, these air pockets contract and the coolant is sucked to replace air, causing bubbles.

It can explain air pockets in your coolant, which may not even be accompanied by overheating.

How To Fix It?

It is a relatively easy fix if your car’s coolant reservoir is boiling due to trapped air.

All you need to do is unscrew your coolant reservoir and start the car.

Let your car run until the fan comes on and turn your aircon to its warmest temperature and turn its fan to full speed.

The bubbles in the coolant will be visible in the coolant at first. Eventually, the coolant will start coming in smoothly.

Afterward, you may notice that your coolant levels have gone down as the air trapped in it is gone. Pour in some more coolant to normalize the levels.

Failing Cooling Fan

The purpose of a cooling fan is to keep the radiator cool while the engine is off. A cooling fan may start to fail due to many reasons, including wiring problems.

Wiring problems usually occur due to a blown fuse or connectors eroded over time. When the fan does not keep the radiator cool, the car coolant reservoir can boil.

A failing cooling fan is characterized by overheating the engine and often a loud noise coming out of the radiator fan.

If the cooling fan motor fails, its blades stop spinning, and airflow is not generated.

How To Fix It?

To fix a failed cooling fan, you will have to get down to the cause and replace the part causing the problem, the relay, motor, a blown fuse, or the temperature switch.

While replacing a fuse or broken wiring is cheaper, replacing a temperature switch can be expensive.

Lack Of Temperature Control

Lack of temperature control happens due to a failed thermostat.

You will know that the reason behind your car coolant reservoir boiling is lack of temperature control when you notice some other things along with the coolant boiling.

It includes high-temperature readings on the gauge, heater malfunction, and coolant leaks.

Since a thermostat regulates coolant flow around the engine, this cause of coolant reservoir boiling can be severe.

A thermostat may fail due to overheating, sludge formation over time, a defect in the thermostat itself, or the thermostat being too old.

How To Fix It?

While your engine may still run and your car may still move, you do not want to drive your car with a failed thermostat. Instead, it would be best if you replaced it right away.

Replacing a thermostat is easy and inexpensive, and you can do it yourself. You may follow any step-by-step instruction video on how to replace a thermostat.

Car Coolant Reservoir Boiling Causes

Blown Head Gasket

The purpose of a head gasket is to seal the combustion gases within cylinders and prevent coolant and engine oils from leaking into the cylinders.

When a head gasket is broken, it can result in compressed air from the cylinders entering the cooling system as they are being blown into the water jacket.

As a result, bubbles may form in the coolant reservoir, which is why it may appear that your coolant reservoir is boiling.

If your car’s head gasket is blown, it may also cause overheating, white exhaust smoke, and steam under the bonnet.

It is why if your car has a blown head gasket, you should tend to it right away.

How To Fix It?

It would be best to let a car mechanic fix the leak to fix this problem. While this can be expensive, you may also use leak repair fluid at home if you are looking for an affordable solution.

Faulty Radiator Cap

Another cause of coolant reservoir boiling could be a faulty radiator cap. The purpose of a radiator cap is to maintain the appropriate pressure of a vehicle’s cooling system.

When a radiator cap is damaged, air could get into your vehicle’s cooling system.

It may explain why your coolant liquid has air trapped in it, causing your car coolant reservoir to appear to be boiling. Furthermore, your engine will start to overheat.

Other indicators of a faulty radiator cap include leaking coolant, overflowing coolant reservoir, low coolant level, and white streaks on the radiator.

How To Fix It?

The solution to a faulty radiator cap is also to replace it. However, one thing to keep in mind is to allow your engine to cool down before you remove the radiator cap.


It is normal and expected for all metal components of your car to gather rust and other contaminants over time. The radiator is the same.

Other than time and age, other factors such as improper flushing can also cause contaminants to build up.

When rust and other debris build up on the radiator, they can block its tubes and hinder the proper functioning of the thermostat and other parts.

The thermostat not working correctly can cause the coolant to boil, which is why your car coolant reservoir may be boiling.

How To Fix It?

You will have to clean your radiator thoroughly and properly to fix this. You may also deep clean your radiator using a bucket of warm water and a few drops of dishwashing soap.

Using a sponge, you may wipe your radiator’s exterior clean and as inward as possible. You may use a dry microfiber cloth to mop up any leftover residual dust.

After you have cleaned your radiator, you should make sure to dry it thoroughly. It is because any leftover moisture could cause rust.

To get rid of rust from your radiator, you may use white vinegar as it works great on all surfaces.

Final thoughts

Once you have narrowed down the cause behind why your car’s coolant reservoir is boiling, it is an easy fix from there.

What is important to note is that some of these reasons can be bad for your car. It is why if your car’s coolant reservoir is boiling, you should narrow down the cause and tend to it right away.