Vehicle engines rely on cooling systems that use either liquids or air to keep them running efficiently. Having said that, the liquid cooling system is standard in most automobiles. The system’s essential component is the interconnected system of pipes that transport engine coolant.
The coolant reservoir is a rubber tank that stores the coolant and is responsible for distributing it to various engine parts.
What causes bubbles in the coolant reservoir?

The coolant reservoir becomes aerated due to the presence of air. Just how? A sealed hose system that permits coolant to circulate around the engine is a common component of pressurised vehicle cooling systems.
Air pockets are created when air enters this confined system. The coolant boils inside the reservoir as a result of the air pocket impeding the liquid’s flow and the subsequent increase in temperature.
Reservoir bubbles can be serious, but they’re also usually nothing to be alarmed about because air will naturally leak into the cooling system as a result of certain actions. Alternatively, if air is present in the cooling system, it may indicate a malfunctioning part that needs fixing. We should examine them.
- Faulty radiator cap
- Air pockets
- Bad thermostat
- Leaky or blown head gasket
- Faulty heater control valve or hose
- Leaky coolant reservoir hose
- Faulty water pump
- Rust and contamination
How do you fix the air bubble in your coolant?
Fixing an issue usually involves getting to the bottom of what’s causing it, as a general rule. So, what to do when the coolant reservoir starts to bubble? Ok, let’s check it out.
- Allow air pockets to escape after refilling the coolant
- Replace faulty thermostat
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.
Conclusion
The presence of bubbles in a coolant reservoir may indicate a major problem or it may be completely harmless. When you turn off your vehicle, the cooling system stops working, but the engine is hot, which means the reservoir could boil. After flushing the cooling system with coolant, even air can become trapped inside.
The cooling system can be compromised if engine or cooling system components are defective. You should inspect the radiator cap, head gasket, and other components on your own or get a mechanic to do so. And before it causes severe engine damage, address whatever is causing the reservoir to bubble.