The car’s cooling system is designed to be a closed system. In other words, nothing should get in or out. However, different failures can create problems with the movement of fluid, or leave air bubbles in the system. What does it mean when there are bubbles in the coolant?
I will explain the cooling system with a description of the major components, so you understand what is going on. I will also suggest what the problem could be for you to solve.
Car Cooling System Basics
To understand the origin of bubbles, it is necessary first to understand how the system works.
Your vehicle’s cooling system comprises many parts. The water pump is the component that circulates coolant to the motor. There’s also a thermostat that regulates the coolant temperature and prevents the cooling fluid from freezing or boiling.
You probably remember the radiator, which cools the coolant before it goes back into the engine. The coolant cools by allowing outside air to flow through the radiator as the vehicle is moving.
There is also a cap at the top of the radiator to maintain a certain level of pressure in the cooling system. If this pressure is lost, the boiling and freezing points of the coolant will change significantly.
This is the basic setup of all cooling systems—the engine basically runs a continuous loop. The coolant goes through a series of hoses to the engine and absorbs some of the heat the engine generates.
Then the coolant is routed back to the radiator to cool down, while the process continues, as long the vehicle is running.
Reasons Bubbles are in Coolant Reservoir
If you see air bubbles in the reservoir, most of the time this is normal. If you are seeing a lot of bubbles and are also noticing issues like overheating, it may be a broken pressure cap, blown head gasket, leaking coolant, or trapped air pockets.
Here is a more detailed list of why you may see bubbles in the coolant reservoir:
#1. No issue at all.
People often worry too much when they find some bubbles in the coolant reservoir due to hearing horror stories about a blown head gasket causing this.
If you only see a few bubbles while topping off the coolant system and you have not noticed anything else wrong with the cooling system (like overheating), you probably don’t have anything to worry about.
However, if there are a lot of bubbles along with a full or overheated system you have some type of issue going on.
#2. Broken Pressure Cap.
To maintain system pressure, there’s a specific cap designed to fit the radiator perfectly. The cap is essential to maintain the correct amount of pressure within the system. If the cap is damaged or broken, air can leak into the system where it does not belong.
Sometimes you’ll get a few bubbles forming near the pressure cap. Also, sometimes it will overheat if the temperature of the coolant cannot be completely reduced. Luckily, this is one of the easiest to fix problems – simply replacing the cap.
#3. Blown Head Gasket.
At the opposite end of the spectrum is a blown head gasket, which no one wants to encounter. When a head gasket blows, air pressure from the coupling can leak out, reducing the boiling point of the coolant.
The coolant will boil over and your engine will run hot too. If it’s not tended to fast, the engine can be severely damaged.
Sometimes, this blow out will be due to the fact the engine was already running too hot, that is why you should always halt drivability immediately when the motor temperature rises above the normal zone.
If the head gasket has blown, you will see air bubbles or oil in the coolant. You may also be able to see coolant in the oil depending on the point of failure in the head gasket.
#4. Air Pockets.
Air pockets can form in the system right after a coolant flush. When coolant is added, air gets trapped, causing a circulation backup in the motor.
If there is air trapped you will see bubbles in the radiator. The engine may also start to overheat. Air can be bled out of the cooling system but is best done by a professional.
#5. Coolant Leak.
Coolant leaks not only allow coolant to leak from the system, but they also permit air to enter the system. Common locations for coolant leaks are broken gaskets or hoses.
The cooling system is made up of a variety of hoses, all of which need to be sealed. If the hose ages, it can become easily damaged and break. Even a small crack or hole in a hose can be a nightmare that has to be resolved immediately.
Not only will coolant leak out of this hose, causing the coolant level to be too low, but it will also cause the engine to overheat. The only solution to this problem is to replace the hose.
#6. Broken Thermostat.
The thermostat controls how much coolant flows in and out of the engine. It opens and closes at certain times based on the coolant temperature.
If the car starts to overheat due to a bad thermostat, you may see bubbles in the coolant. Replacing the thermostat is generally not very pricey, normally averaging $150 to $400 depending on a couple of things such as parts or install.
#7. Malfunctioning Water Pump.
The water pump circulates the coolant through various hoses and pipes at all times when it`s running so that the coolant can fill everywhere in the system.
If the water pump does not work, the coolant will not circulate through the engine, and it will likely run too hot. Bubbly coolant is a problem in and of itself.
Additionally, air can get trapped in the water pump, but won’t escape unless you remove the water pump itself.
#8. Defective Heater Control Valve.
The heater control valve, which allows hot coolant into the cabin to heat it.
Hot coolant flows from the engine that can be harnessed by the climate control system to warm you up on a cold winter day. It’s an effective means to re-route heat to its best use.
If the heater valve is bad, it can allow air to enter into the system, causing air bubbles to become a problem. It also could cause the engine to overheat.
Worst of all, the passengers won’t get the heat they need, so they are left cold in the dead of winter.
#9. Rust/Contamination.
A metal radiátor will corrode regardless of how much care is provided to the car. But if you don’t take care of it, it will corrode faster. Thus, it is well-advised to have that radiator cleaned frequently.
Debris can block the radiator which can create damage that is completely avoidable. You must also ensure you have the proper level of coolant in the system.
When the radiator is compromised because of the degradation, or full of debris, the coolant can’t disperse heat efficiently. Inside the system, the coolant will boil and create bubbles in the radiator tank.
The coolant boiling can also entail an overheated engine leading to possible engine destruction.
Why is my coolant bubbling but not overheating?
Coolant can bubble without overheating because you may have air trapped in the system, a bad coolant pressure cap, or a head gasket leak that isn’t large enough to overheat an engine.
How do you know if your head gasket is blown?
To know for sure you need to have a mechanic look at it. However, common signs are white smoke from exhaust, overheating which is unusual, or oil found in the coolant and coolant in the oil.
How do I know if I have air trapped in my cooling system?
The only way to absolutely know is to flush the system or properly bleed it. Similarly, bubbles rising in the reservoir or if your engine temperature fluctuates, it may indicate trapped air in your cooling system.
Can air bubbles in coolant cause overheating?
Yes, air pockets or a bad head gasket can cause your engine to overheat.
Overhearing an engine can entail very costly repair bills, so you absolutely want to have it fixed as quickly as you can.