What causes Bubbles in Coolant Reservoir? and How to Fix it

Automobiles depend on either a liquid or air cooling system to keep the engine running effectively. However, most vehicles utilize the liquid cooling system. This system relies on a close network of pipes that carries coolant to the engine.

This coolant is stored in a rubber tank called the coolant reservoir; from here; it dispatches coolant to engine components.

However, at a point coolant reservoir may start boiling, and you may wonder why. Bubbles in a coolant reservoir could suffice for different reasons. While some may not need you to worry, others will require you to take immediate action.

What causes bubbles in the coolant reservoir?

what should be done if the coolant reservoir is bubbling

The air causes bubbles in the coolant reservoir. How? Many vehicle cooling systems are pressurized and use a sealed hose system that allows coolant flow around the engine. When air finds its way into this closed system, it forms air pockets. The air pocket blocks the flow of liquid, which leads to a rise in temperature, causing coolant to boil inside the reservoir.

Bubbles in reservoirs could be severe, and sometimes, it could be something not to worry about because some actions will naturally cause air to sip into the cooling system. In other cases, the air in the cooling system could mean there is some faulty component that needs to be addressed. Let’s take a look at them.

Air pockets

When coolant is flushed and refilled, air pockets are trapped in the cooling system. As you drive the car and it heats up, the air pockets expand and are pushed into the coolant reservoir. As the engine cools down, the expanded air pockets contract; hence, coolant is sucked from the reservoir to replace air. Thus causing the bubbles. This is normal but can be avoided.

Faulty radiator cap

The radiator cap works as a pressure seal to keep pressure in the cooling system high while letting coolant flow into the expansion tank. When this cap is faulty or replaced with a non-compatible one, it allows air into the cooling system, causing the coolant reservoir to boil. 

An easy way to know bubbling is due to a bad radiator cap is that you will see bubbles in the coolant reservoir after driving for a long.

Bad thermostat

The thermostat regulates the flow of coolant in and out of the radiator. Here, the thermostat must open and close at a specific time. It should open to let out the hot coolant from the engine into the radiator to cool. Then, remain close for the engine to heat coolant to start the car.

When the thermostat is faulty, it doesn’t open and closes at the appropriate time. Thus, it causes a bubbling effect in the coolant reservoir or radiator since there is an uncontrolled airflow.

Leaky or blown head gasket

A worn or blown head gasket is a prevalent cause of bubbles in the coolant tank. When the head gasket is bad, combustion gases from any cylinder slip into the water jacket, then send air through the cylinder head into the cooling system.

An easy way to know the bubbling is due to a leaky gasket is that bubbling occurs immediately after starting the engine.

Faulty heater control valve or hose

The heater control works to keep the passenger compartment hot by allowing hot coolant to enter the heater’s core. When the heater’s valve is faulty or the heater’s hose connection is loose, the air slips into the cooling system and results in coolant boiling in the reservoir.

Leaky coolant reservoir hose

The hose that connects the radiator to the coolant reservoir must be leak-free and tight at all times if the radiator hose is bad or worn, air sips into the hose and causes the coolant tank to boil.

Faulty water pump

The water pump helps circulate antifreeze or coolant through the radiator’s cooling tubes, where it cools and flows to the engine components through the engine paths and pipes. When the water pump is bad, air enters the pump intake. When the trapped air reaches the radiator intake or coolant’s valve, it generates bubbling.

Rust and contamination

Like other metal components, the radiator can rust with time. In addition, when a radiator is not flushed regularly and supplied with appropriate coolant, it gathers impurities. 

Here, rust, grime, and other debris can block the radiator’s tubes and distort the smooth operation of the thermostat and water pump. Excessive heat lets the coolant boil and produces fast bubbles you see in the coolant reservoir.

How do you fix the air bubble in your coolant?

As a natural rule, one way to fix a problem is always to tackle the root cause of the problem. So the question is, what should be done if the coolant reservoir is bubbling? Let’s take a look.

what should be done if the coolant reservoir is bubbling

Allow air pockets to escape after refilling the coolant

When flushing and refilling the coolant reservoir or radiator, the air is trapped in the system. Therefore you should ensure the air leaves. To remove air pockets, run the engine with the radiator’s cap off for fifteen minutes after refilling. This will help bleed any air in the radiator or reservoir before replacing the cover.

Replace faulty thermostat

With the thermostat malfunctioning, the coolant flows inappropriately, causing the coolant to boil. The best way to avoid this is to replace the thermostat with an OEM or compatible aftermarket one. So long the thermostat is working fine, coolant movement will be regulated, and there won’t be bubbling. 

Fix a leaky head gasket.

Since a leaky head gasket causes combustion gas to enter the cooling system, one needs to seal the leak. To

do this, take the car to a professional mechanic to fix the leak. You can fix it at home using a leak repair fluid if you want to save a few bucks. However, if the leak refuses to seal, the best option will be to replace the head gasket.

Head gaskets are fixed in positions where one would have to dismantle almost half of the engine to get to it—as such, replacing it is quite an expensive fix. One reason you shouldn’t allow the head gasket to get to this point. And if you decide not to fix it, it can lead to permanent engine damage.

Replace or seal the radiator cap.

With the radiator cap out, the pressure starts fluctuating and can damage the radiator or allow air into the cooling system. Here, you will need to change the radiator cap; ensure you replace it with an OEM or manufacturer-recommended cap.

An incompatible radiator cap will also allow air into the cooling system.

Replace or seal the coolant reservoir hose.

When the reservoir hose is leaking, something you don’t want enters, and something you need leaves. So changing the hose quickly will be a wise thing to do. Because aside from air entering the cooling system through the leaky hose, coolant also drips from the leaky hose. So you still get to experience low coolant levels leading to engine overheating. 

Replace the heater control valve and hose.

Aside from air slipping into the cooling system. Vehicle occupant still suffers from it. As such, you should replace the heater valve and tighten the hose. If the hose is worn, you will still need to change it.

Change the bad water pump.

The water pump is supposed to remain closed. But when the cap is bad, it traps air in the pump intake. The best thing here will be to replace this cap. With the cap closed, no air is trapped inside the water pump.

Flush coolant reservoir and radiator

When coolant works to an extent, it becomes dirty, forming sludge, grime, and other particles. This causes the radiator to get clogged, not allowing other connected components to function well. So ensure you regularly flush the radiator or coolant reservoir and refill it with a new coolant.

Frequently Asked Questions (FAQs)

Is it normal for the coolant reservoir to bubble?

Bubbling in a coolant reservoir sometimes is normal.  A coolant reservoir bubbling when the car is off is an excellent example of a normal bubble. How? When you park your vehicle and turn off the engine, all cooling systems will shut down. But the engine will still be hot. 

Meaning the water pump, cooling fan, and every other cooling component stopped working. But coolant will still be circulating in the engine. The heat that builds up in the engine with nothing to cool it down at this moment causes the reservoir to bubble. But this is only temporary, so you have nothing to worry about.

Can a bad thermostat cause bubbling in the coolant reservoir?

A bad thermostat can cause bubbling in the coolant reservoir. The thermostat ensures the coolant stays where it is needed and leaves when necessary. But when it goes bad, there will be an unregulated flow of coolant, which causes bubbles in the reservoir.

Can a bad water pump cause coolant to bubble?

Yes, a bad water pump can cause coolant to bubble. The water pump helps circulate coolant through the radiator tubes, where it cools. The cool coolant then spreads into the engine components through the engine’s pipes or passage. 

With the pump bad, there won’t be anything to circulate coolant to the radiator. First, the engine component won’t get enough coolant for proper lubrication to function well.  Secondly, since the circulating coolant doesn’t get to cool, it will cause engine overheating, which in turn causes a bubble in the reservoir.

Can an air bubble cause a car to overheat?

Air bubbles can cause a car to overheat, depending on where this bubble is trapped.  Naturally, you will find bubbles in the coolant reservoir. But the presence of bubbles in the reservoir slows the rate at which the coolant absorbs heat. 

Since coolant does not absorb the engine’s heat on time, the engine builds up heat. This build-up of heat that doesn’t cool on time causes the engine to overheat. If this air happens to be in the radiator, it blocks coolant from adequately circulating in the engine. So long the engine does not get coolant to cool it down when hot, it overheats.

Why is my coolant boiling but the car not overheating?

Sometimes, coolant bubbling in the reservoir but not overheating the engine is normal. Here, it’s most likely that what your car needs might be a light repair. It could be a result of contaminated/low coolant or a bad air radiator cap. You may need a proper diagnosis to reveal the root cause.

Try replacing the radiator cap. You can also top the coolant or do a complete coolant flush, then replace it. Afterward, drive the car. If the bubbling stops, the cause is a faulty radiator or contaminated/low coolant. If the bubble still occurs, take the vehicle to a mechanic.

Final words

Bubbles in a coolant reservoir could mean something serious, and at the same time, it could be something not to worry about. The reservoir can boil after you off your car; the reason is with the car off, the cooling system shuts down, but the engine is still hot. Even air can be trapped in the cooling system after a coolant flush.

However, faulty engine or cooling system components can let air into the cooling system. Check the head gasket, radiator cap, and other parts yourself, or have a mechanic do it. And fix whatever causes bubbles in the reservoir before it leads to catastrophic engine damage.

Osuagwu Solomon

Osuagwu Solomon is a certified mechanic with over a decade of experience in the mechanic garage, and he has over five years of experience in the writing industry. He started writing automotive articles to share his garage experience with car enthusiasts and armature mechanics. If he is not in the garage fixing challenging mechanical problems, he is writing automotive repair guides, buyer’s guides, and car and tools comparisons.

2 thoughts on “What causes Bubbles in Coolant Reservoir? and How to Fix it

  1. Hi I had minute bubles inside the coolent reservoir and small loss of coolent for a year and without any serious issue.
    A week back I added K-ultimate seal and started to have noticable bubbles in the reservoir, in addition to pressure in the water circulating pipes and inside the reservoir tank.
    Any indication of minor or a serious issue and how to tackle the problem please.
    Thanks

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