Auto Diagnosis: What Causes Air Bubbles in Coolant Reservoir and How to Fix the Issue

Air Bubbles in the Cooling System

Ventilation of clean air through a car engine is good for its performance as it helps maintain proper combustion. However, the real problem arises when excess air gets into critical vehicle systems, such as fluid lines. 

Hindering the flow of cooling fluids, trapped air can create hot spots in the engine. This may lead to overheating and other internal engine issues, causing significant damage. However, timely diagnosis and fixing can help avoid potential hazards and costly repairs.

Let’s find out more about the problem of air bubbles in the cooling system. This car trouble is also known as a radiator airlock.

Some Common Reasons for Air Bubbles in the Cooling System

Here are some of the common causes of air bubbles in the car cooling system and their fixes.

Air Pockets

Air can get trapped in the cooling system when the fluids are drained and replaced, during a radiator flush. The trapped air then expands as the vehicle warms up and is forced into the coolant reservoir. 

However, when the engine cools, the expanded air pockets collapse and the coolant is pulled back. This creates a void and results in the appearance of air bubbles. 

How to Fix It

An effective way to get rid of air bubbles in the coolant reservoir is to perform the radiator flush again. Attempt it carefully this time by using the proper tools and techniques. Seek professional help if needed to prevent any excess air from entering the system. 

Regardless of the type of radiator flush attempted, use personal protective equipment (PPE) to avoid potential safety risks.

Problematic Radiator Cap

bad radiator cap also causes air bubble
If radiator cap breaks or doesn’t properly fit, air can make its way into the cooling system

The radiator cap serves as a pressure seal. It maintains the high pressure in the system while the coolant flows into the expansion tank. If this cap breaks or doesn’t properly fit, air can make its way into the cooling system. As a result, bubbles start appearing in the coolant reservoir and cause your car to underperform.

How to Fix It

It is important to know when to replace a radiator cap to stop air bubbles from disrupting the cooling system. Moreover, getting rid of the faulty or incompatible radiator cap can help prevent the frequent loss of fluids. That said, never touch a radiator cap with bare hands after a long drive. It can get excessively hot and may cause serious burns. On a side note, you can follow this step-by-step process to fix radiator leaks.

Malfunctioning Thermostat

A bad car thermostat is among the common reasons for a radiator airlock. When a thermostat malfunctions, it fails to open and close at proper intervals. This results in disrupted lower coolant levels and disrupted fluid circulation, creating air bubbles in the system.

How to Fix It

When facing the coolant reservoir tank bubbling issue, check the thermostat for signs of damage or wear. After carefully examining its condition, determine whether it needs to be repaired or replaced. 

If there is too much air in the coolant reservoir, get it flushed while getting the thermostat fixed. Test the newly restored system by letting the engine run for a while to see if it overheats or not.

If everything is fine and the temperature gauge still gives an erroneous reading, it may indicate other underlying problems. Check for these common causes of the fluctuating temperature gauge and fix them if required to get actual readings.

Leaking or Blown Head Gasket

air bubbles in the coolant system are caused by blown head gasket
Blown head gasket causes visible bubbling at the coolant reservoir

A failing head gasket is among the most common causes of air bubbles in the coolant tank. When the head gasket blows, it causes combustion gases to leak into the water jacket. 

This way the air is forced into the cooling system through the cylinder head. The visible bubbling at the coolant reservoir soon after the engine starts is a prominent symptom of a blown head gasket.

How to Fix It

There are gasket sealers available in the market, but they may only temporarily solve the problem. However, it is recommended to replace the failing parts and prevent further damage. Replacing a head gasket may be expensive, but it will be a long-term solution that may prevent other coolant tank bubbling causes.

Whether replacing or repairing a gasket, both the engine oil and coolant need to be completely drained. This will release all the trapped air in the system. There are different types of head gaskets, make sure to choose the right one when replacing it.

Bad Water Pump 

bad water pump is a cause behind air bubbles in coolant reservoir
A malfunctioning water pump hinders the fluid flow in the cooling system causing air to trap in the system

A water pump circulates coolant and other fluids throughout the system via engine pipes. When a water pump malfunctions, the air doesn’t need the pump input to sneak into the system. As a result, the trapped air makes bubbles when it gets to the radiator intake or coolant valve. A failed water pump is also one of the reasons behind car radiator water leaks.

How to Fix It

Look for signs of overflow and splotches to inspect the condition of the water pump. If the seals are damaged, replace the water pump to get rid of the radiator airlock problem. If available, check the car user manual for instructions and attempt the aforementioned procedure carefully.

Burping the Cooling System to Fix Air Bubbles

removing air bubbles from cooling system through burping
Burping is a process that helps release air bubbles in the car cooling system

A process called “burping” can also help resolve the issue of coolant tank bubbling Here’s how to attempt it.

  • Ensure the engine is cool before proceeding.
  • Remove the radiator cap carefully.
  • Start the engine and let it run.
  • Allow the engine to reach operating temperature. This will cause the thermostat to open and the coolant to circulate.
  • During this process, air bubbles trapped in the system will be released.
  • Keep an eye on the coolant level as the air bubbles are being purged.
  • Top up the coolant as needed to maintain the proper level.
  • Once the coolant is circulating smoothly and no more air bubbles are being released, carefully replace the radiator cap.

If unsure about performing any of the aforementioned steps, seek assistance from a certified auto technician. Besides a radiator airlock, engine overheating is a serious problem. Consider checking and replacing the coolant temperature sensor if it’s faulty or damaged.

These were some of the most common causes of air bubbles in the cooling system and their fixes. While inspecting the vehicle cooling mechanism, check for these signs of a failing radiator fan. If spotted, try to fix them right away to ensure optimal performance of the cooling system.

When buying a used car in the UAE, diagnose its engine cooling system for the aforementioned problems. Maintaining an efficient cooling system is crucial, particularly when driving in the UAE’s hot climatic conditions. Doing so will help you avoid encountering damaging and costly engine issues.

For more car maintenance guides and troubleshooting tips, stay connected with the dubizzle auto blog.


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