Car cooling systems rely on a sealed network of hoses to pump coolant/antifreeze around an engine.
These hoses are arranged in a sealed loop, which allows a smooth and constant flow of coolant/antifreeze to reach components including the head gasket, cylinder heads and engine block – keeping the engine working at the right temperature.
When air is allowed to enter this sealed system, blockages and bubbling can occur, which can lead to the engine overheating.
There are several possible causes for air pockets in the cooling system, and we’ll cover these as part of our guide on what it means when coolant/antifreeze bubbles in the system.
Why is my car’s coolant bubbling?
Cooling systems in most cars are pressurised, and rely on a leak-free closed circuit of hoses to pump coolant/antifreeze around the engine. When air gets into this sealed system, air pockets can form and cause blockages, which can lead to bubbling and overheating.
Bubbling indicates rising air pressure in the cooling system, which is a sign that the flow of liquid is blocked by a pocket of air.
One of the most common causes is a blown head gasket, in which the air pressure inside the cylinder heads is transferred to the cooling system. This escaped air causes bubbling in the coolant/antifreeze reservoir, which can often be mistaken for boiling.
Aside from a blown head gasket, there are several other causes for air inside in the cooling system, which we’ll come to later.
What causes bubbles in the coolant reservoir?
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
Air pockets are trapped in the cooling system when coolant is drained and replaced. As the vehicle warms up, the air pockets expand and are forced into the coolant reservoir.
Because the expanding air pockets collapse when the engine cools, coolant is pulled from the reservoir to replace the air. As a result, the bubbles appear. This is a common occurrence, but it may be prevented.
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
A malfunctioning car thermostat controls the flow of coolant into and out of the radiator. The thermostat must open and close at particular times in this case. It should open to let the hot coolant from the engine escape and cool in the radiator.
Then stay near to the car as the engine heats the coolant to start it. When a thermostat malfunctions, it fails to open and close at the proper times. As a result of the unregulated airflow, it generates a bubble effect in the coolant reservoir or radiator.
Leaky or blown head gasket
The most common cause of bubbles in the coolant tank is a worn or blown head gasket. When the head gasket fails, combustion gases from any cylinder leak into the water jacket, forcing air into the cooling system through the cylinder head.
The presence of bubbling soon after starting the engine is a simple method to determine whether the bubbling is caused by a faulty gasket.
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
A faulty water pump circulates antifreeze or coolant via the radiator’s cooling tubes, where it cools and goes to the engine components via the engine pathways and pipes.
When a water pump malfunctions, air enters the pump input. The trapped air causes bubbling when it reaches the radiator intake or coolant valve.
Rust and contamination
The radiator, like other metal components, can rust over time. Furthermore, if a radiator is not cleansed and supplied with the proper coolant on a regular basis, it will collect pollutants.
Rust, filth, and other debris can clog the radiator’s tubes, causing the thermostat and water pump to malfunction.
Signs of air bubbles in the coolant system
Here are the main signs of coolant bubbles in the reservoir:
Visible Bubbling or Foaming
The most obvious sign is visible bubbling or foaming occurring within the coolant reservoir or expansion tank.
Trapped air will agitate the coolant, reducing its normal clarity and causing it to appear excessively turbulent on the surface. The bubbling may appear steady or intermittent depending on how much air is present and where it is entering the system.
Fluctuating Coolant Levels
As the engine warms up, air pockets dissolved in the coolant will precipitate out, pushing air up and out of the system. This causes the coolant level in the reservoir to rise as pressure increases and also interrupts the coolant circulation.
Owners may need to top off levels more frequently than normal due to the expanding air taking up space in the cooling loops.
Higher Temperature Readings
The infiltration of air inhibits the coolant’s ability to absorb and transfer heat efficiently. This allows engine temperatures to exceed normal operating ranges. A reader may display lower than actual temperatures due to air insulation within the system.
Steam From Reservoir or Radiator
As air is pushed out by pressure, it may appear as visible steam escaping through the coolant overflow bottle or radiator during and after driving. The steam is caused by the rapid boiling and condensing of coolant as it circulates.
Loss of Coolant
As air continuously mixes with the liquid coolant, it displaces the actual volume of fluid circulating through the cooling passages. Over longer periods, this cumulative effect from ongoing air infiltration can manifest as a noticeable drop in the overall coolant level contained within the system.
Refilling may be required more and more frequently to compensate for the air’s invisible yet volumetric replacement of the coolant. A coolant shortage developing gradually is a red flag that air is leaking in somewhere and needs resolution.
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.
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.
5 steps to bleed the air from your car radiator
1. Turn off the engine
Your engine needs to be completely cool before you begin. The last thing you want is to suffer burns from hot coolant.
2. Remove the radiator cap
The radiator cap is typically located at the top of the radiator. In most modern cars, this cap is situated at the highest point of the coolant system so air bubbles naturally accumulate there.
To remove the radiator cap, first wrap it with a thick rag and give it a half-turn anticlockwise. This will allow any residual pressure to escape.
Your radiator may have an overflow tank
Some vehicles have an overflow (also called expansion) tank connected to the radiator. If yours does, remove the cap from the overflow tank using the same method as above. If you have an overflow tank, you’ll be bleeding air from the tank rather than from the radiator directly.
3. Top up your coolant system
Place the funnel into the radiator filler neck or overflow tank. Use the correct adaptor to ensure a tight seal so the funnel is secure. Slowly pour the coolant into the funnel. Pour with a steady, slow stream to avoid adding any air bubbles.
Keep pouring until the coolant level reaches the ‘full’ marker on the inside of the overflow tank or until it brims the radiator. Leave the funnel in place. You can try squeezing the top radiator hose to expel any air trapped inside it.
This is an optional step since a bleed valve is not included on all radiator types. It looks like a little screw sticking out of the radiator, but its location differs by make and model. You may be able to use your owner’s manual to locate your car’s bleed valve.
The purpose of the bleed valve is to bleed air and release pressure from the cooling system. To use the bleed valve, loosen it (but do not remove it completely) and then begin to top up your coolant.
As the added coolant enters the system, you may hear air escaping from the valve. Then, coolant will start to sputter out of it. Keep topping up slowly. When the coolant starts to pour out of the bleed valve in a steady stream, tighten the valve again.
At this point, your system is full and has little or even no air in it.
4. Run your engine and burp the coolant
Clear away any tools or rags from around the engine, leaving the funnel in place and secure.
Turn on your car’s engine and set the A/C to its hottest setting. This will allow the coolant to cycle through the engine and entire cooling system while bringing any trapped air to the top.
Any air in your system should bubble up and ‘burp’ out of the funnel. You will need to keep your engine running for a few minutes. Rev your engine to about 3,000 RPMs occasionally to help dislodge and push out any trapped air.
Keep a close eye on the coolant level in the funnel. It may rise, as coolant expands under heat and pressure, or it may drop as it cycles and air bubbles escape.
If your coolant level drops below full, top it up with more coolant and continue the burping process.
5. Shut off the engine and top upof the coolant
Your coolant level should stop fluctuating and even out. Once that happens, turn off the car engine and let it cool down completely. Once your engine is cool, your coolant level might drop. Top it up taking care not to overfill.
Remove the funnel and replace the radiator cap.
Pro tip: If you’re using a professional radiator funnel kit, it should come with a funnel stop. You can use it to block the funnel before removing it from your radiator, so that no fluid spills. If you don’t have a funnel stop, be ready to catch any spills.
Cost to fix coolant bubbling
The potential cost to fix coolant bubbling in a vehicle can vary based on the specific issue and the cost of parts and labor in your location. Here are rough estimates for common repairs:
- Diagnostic Fee: $50 – $150 (may vary by location and shop).
- Thermostat Replacement: $100 – $200 (parts and labor).
- Water Pump Replacement: $150 – $300 (parts and labor).
- Head Gasket Repair: $1,000 – $2,000 or more (parts and labor).
- Coolant Flush: $50 – $150 (parts and labor).
- Radiator Replacement: $200 – $500 (parts and labor).
These are general estimates, and actual costs can vary. It’s crucial to get a specific quote from a reputable mechanic after they diagnose the exact cause of the coolant bubbling in your vehicle.
Additionally, prices can differ based on the make and model of your car and the local labor rates.
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.