Bubbles In Coolant Reservoir And In Coolant: Is That Okay?

Every vehicle’s cooling system is dependent on the sealed network of hoses, and it pumps the coolant around the engine. These hoses are placed in a sealed loop. This will allow and deliver a smooth and constant flow of coolant or antifreeze. 

And this will then reach the components that include the head gasket, the cylinder heads, and the engine block. This will also keep the engine operating at the right temperature. However, if there is air in the sealed system, there can be bubbling or blockages. 

This can also lead to overheating. There are many reasons why there are air pockets in the cooling system. 

In this article, we will discuss why there are bubbles in your coolant reservoir and if it is okay. Let us start. 

Bubbles In Coolant Reservoir And In Coolant

What Causes Coolant To Bubble In The Reservoir?

The cooling system of the vehicles is pressurized. They have a leak-free closed circuit of hoses, and they will pump the coolant around the engine. If there is air in the sealed system, the air pockets will form, and it will then cause blockages. 

This can also lead to overheating and bubbling. The bubbles mean that there is rising air pressure in the cooling system. This is a sign that the liquid flow is blocked by air. 

One of the most common causes of a bubble in the coolant is a blown head gasket. This is where the air pressure inside the cylinder head will be transferred to the cooling system. The air that has escaped will cause bubbling in the coolant. 

People also describe it as boiling. However, aside from a blown head gasket, there are still other reasons for the bubbles, but the blown head gasket is the most common. 

Is It Normal For Coolant To Bubble?

Yes, it is expected if you can find some bubbles in the overflow tank. One of the overflow tank jobs is to remove all the bubbles caused by the air in the cooling system. The automotive coolant will wear more efficiently even without the air bubbles. 

The air bubbles will give the coolant more allowance to absorb heat at a faster rate too. But though it is normal to find some bubbles in your coolant and the overflow tank, the bubbles can also signify a leak in the head gasket. You can test this by checking all the cylinders of the vehicle. 

Use a cylinder leakage tester and keep the engine off. If the bubbles are present during the test, the combustion gas will leak into the cooling system, which will need to be repaired. 

How Do You Fix Bubbles In The Coolant?

One of the options is to have it fixed by a professional. It can be expensive, and so some people are just using a leak repair fluid. You can have a safe and permanent repair. 

There are instructions to follow. If there is a fault in the pressure cap, the pressure cap will be replaced before the change of the pressure so that damage to the radiator can be avoided. You need to ensure that the cap is replaced with an OEM or an original equipment manufacturer component. 

You can go directly to the manufacturer. During the flushing of the radiator and the cooling system, you also need to make sure that you will run the engine with the pressure cap for fifteen minutes. 

Why Is My Coolant Bubbling But Not Overheating?

It is normal to have bubbles in the overflow tank, and the engine is not overheating. A bubble in the coolant is not an indication of a bad coolant or a low coolant. 

Therefore, the coolant can still function and do its job of preventing your engine from overheating. That is why your engine is not overheating even with the bubbles, 

Tiny Air Bubbles In The Radiator: Is That Okay?

There are situations where it is okay, but there are also cases where the bubbles can mean more severe damage. The air bubbles in the coolant, radiator, or expansion reservoir mean that the air has entered the system. This can lead to overheating. 

What Causes Air Bubbles In The Radiator?

If there are air bubbles on the radiator, the air has entered the system at some point. This creates air bubbles in the radiator. 

How Do I Get Rid Of Air Bubbles In My Radiator?

You can slowly remove the cap of the radiator. Next, also remove some coolant and ensure that it does not bubble up. 

You then have to run the engine and then let it warm for a while. If you have achieved the operating temperature, the valve of the heater will then automatically let the coolant flow into the heater; this will then push out all the air bubbles. 

Radiator Bubbling With Cap Off: Is That Normal?

Yes and no. There are situations where the bubbles in the air are normal, but if you have a malfunctioning radiator cap seal, it will allow some air to enter the system. This will produce some bubbles in the expansion reservoir. 

Can Bad Thermostat Cause Bubbles In The Coolant Reservoir?

Thermostats will open fully so that they can allow full coolant flow during normal driving conditions. The thermostat can then close or jam. 

This will then lead to overheating or undercooling. If you have a faulty thermostat, it can cause an irregular opening or closing and cause churning and bubbling. 

Can A Bad Water Pump Cause Coolant To Bubble?

If you have a bad water pump, it will allow air into the pump. After that, there will be bubbling if the air is trapped into the inlet neck. 

Coolant Bubbling After Shutdown: Is That Normal? What To Do?

The bubbling means that there is air pressure in the coolant. The cause is typically a blown head gasket. 

The engine will not run because the compression will lower. After all, the cylinder will lose its seal. This is not normal. Consult an expert immediately.


In summary, coolant keeps our engine cool and prevents it from overheating. That is why you need to keep your coolant at an optimum level. People report that they have some bubbles in their reservoir. 

This happens if there is air inside. Sometimes this is normal. Sometimes it can also be a sign that something is wrong.



Image credits – Canva

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My name is Hank, and I've been in the automotive industry for 27 years. I've been working in my own auto repair shop for the last 13 years, and now I want to help you here, on my blog. Let me know if you have any questions. Read more