These are the causes of air in the engine’s cooling system

A car heater that is not functioning properly, but also an engine that is warming faster, which may cause its overheating, are some signs of air in the engine’s cooling system.

The most common causes for air formation in the cooling system are insufficient coolant level and depressurization of the system, but there are also others.

If you ever saw a heater that blows cold air, either at idle or when driving is a consequence of air in the cooling system.

Why does air form in the cooling system?

In an engine that is functioning as it should, air that enters the cooling path is squeezed out through the coolant’s expansion tank, but if there is a problem with the system, air may form, which impedes the movement of coolant. 

The reasons for airing the engine cooling system are most often associated with a violation of its tightness. During heating, the antifreeze expands, and when it cools, it contracts, and pressure drops appear in the circuit. 

To align them, the manufacturer provides a two-way valve in the cap of the radiator or expansion tank. It relieves excess pressure into the atmosphere during strong heating and sucks the air back in when it cools down. 

The reason for the appearance of air in the cooling system lies in the incorrect operation of the valve itself or its constant suction into the system through other nodes.

Causes of air in the cooling system

1. Faulty cylinder head gasket

A damaged head gasket that leaks or one that is not correctly tightened lead to the suction of air into the cooling circuit when the pressure in it decreases. Also, exhaust gases may leak into the cooling circuit.

2. Jammed thermostat

A bad thermostat disrupts the circulation of the coolant, which is why air bubbles trapped in it cannot enter the coolant expansion tank, and from it into the atmosphere.

3. Bad water pump

If there is a leak in the water pump, then it can suck in air through it and air locks will appear in the cooling system. Moreover, if the pump is worn out, it does not create the required pressure and cannot push the air bubbles to expel them into the expansion tank.

4. Clogged or broken valve on the expansion tank cap

This issue can also lead to air forming in the coolant path. That faulty valve leads to the fact that excess air is not released when the antifreeze is heated and the pressure in the circuit rises.

If the valve does not act as it should, then to equalize the pressures during cooling, air cannot enter the tank, which increases the risk of suction in other places.

5. Leaking radiator or heater

When cooling, air can penetrate through cracks found in the radiator or heater.

6. Low coolant level

When the coolant level drops under the minimum required levels, due to leaks or water evaporation, the pump will suck air from the expansion tank.

7. Deposits on the walls of the cooling circuit

If deposits form on the walls of the cooling circuit, it will reduce its cross-section and throughput, leading to a disruption of the coolant normal circulation, which is why air bubbles are not expelled from the system.

8. Worn out pipes, hoses, and seals

Seals that are worn, deformed pipes, loosening of the clamps lead to air leakage into the cooling system when the pressure in it drops.

Symptoms of air in the cooling system

It’s hard to find out if there is air in the cooling system since it is impossible to see them. To know if air has somehow managed to enter, you need to know the indirect signs. These are:

1. White smoke from the exhaust

This is the most common symptom that indicated the coolant has entered the combustion chamber because of a bad head gasket. Also, the same gases from the cylinder block can enter the cooling system through it.

2. Poor heater performance

If you see a heater that is blowing cold air at idle and works when you speed up the engine over 3000 RPM, this is a clear sign that air is present in the cooling system.

3. The engine warms up faster

This is also a sign of air bubbles in the cooling system. Because of this, the cooling process of the engine is disrupted, the engine warms up faster, and the temperature rises above the normal operating temperature.

At the same time, the radiator will remain cold or warm. A common root cause of this symptom is a stuck thermostat in the closed position.

4. Coolant level is low

This is another common cause why air forms.

5. Traces of coolant leaks

Wet spots, dried white smudges on the pipes, engine, reservoir, and other places located next to them are a sign of coolant leaks in the system facilitating the presence of air bubbles.

Now that we have covered the causes and symptoms of air in the engine’s cooling system, we will go more in-depth to know how to find these air bubble reasons on those car parts.

How to know if there is air in cooling system

1. Inspect the radiator and cooling system pipes

Also, check their joints for cracks or other signs of worn. Check the tightness of the clamps, and also make sure that there are no cracks in the hoses.

Sometimes coolant doesn’t leak through small cracks, while air may enter through them when the pressure drops.

2. Check the operation of the water pump

Try listening to its noises and inspecting for coolant leaks. You can check the operation of the pump by pressing with your hand the branch pipe that goes to the radiator when the engine is running, you should feel the pulsations of the liquid inside.

3. Check if air bubbles appear in the expansion tank

When the engine is warm, you can check the presence of air bubbles while speeding up to 3000 rpm. Their presence often indicates a breakthrough of gases from the engine.

If at the same time the coolant level gradually decreases, and white smoke comes out of the exhaust pipe, there is a high probability of a damaged cylinder head gasket.

What happens if there is air in the cooling system?

Air is much lighter than antifreeze, so it accumulates at the high points of the cooling circuit and prevents the liquid from circulating normally. Because of this, the operation of the cabin heater is disrupted and the engine cooling operation deteriorates.


As you saw, there are many reasons why air is present in the engine’s cooling circuit. There are some side effects because of this issue.

Diagnosing and repairing is challenging and requires some knowledge, so it’s best to take your car to a specialized repair shop to have a thorough check for the above signs and causes.

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