Don’t be fooled by the ‘Air Lock’

Installing any engine coolant is a fairly simple process and for many is a DIY process. However, allowing air to get trapped in the cooling system can have negative effects on the overall performance of your cooling system.

On occasion Evans Cooling Systems have received feedback from customers, that after installing Evans, they have noticed a coolant temperature increase of well over 15C.......this is not normal!

Evans can increase coolant temperatures slightly (approx. 5C-7C) but should never increase beyond 15C.

For many, the first reaction is to say, ‘Evans Coolants don’t work!’, however, in many cases the issue was found to be an air lock, created by air which was trapped in the coolant system due to insufficient bleeding during/after the installation process.

The term ‘air lock’ is a generic term used to refer to trapped air, but the name can vary depending on the location of the air.

Evans is a more viscous fluid than water, therefore air can trap more easily during installation and take a little longer to find its way out.

 Here is a brief description on the two most common air locks experienced and how to try rid your cooling system of air.

 Common Air Lock

Located in the cooling system, more commonly in the upper hose, heater core or thermostat (highest points of the system).   A common air lock normally shows up as higher coolant temperatures accompanied by low or no expansion.  In some cases the heater will not function correctly when engine is at operating temperature.  The air pocket will create a hot spot and low levels of coolant will decrease efficiency.

Some engine designs have a specific problem with the way the coolant passages are designed or located making it more difficult to move the air out of the system. For example, in an LS engine, the head is higher than the pump (outlet) creating the need for the bleed lines on the cylinder head.

Normally the OEM will have a specific bleeding procedure to follow eliminating the air.

The simplest remedy for an air lock is cycling the system. Start the engine run it up to temperature, shut it off, allow the engine to cool and repeat as required. Normally 1 or 2 heat cycles will show the level change and topping of indicates the air has moved though the system.  If replacing the thermostat drill 1 or 2 1/8th" holes to allow the air to easily move though the thermostat. Most thermostats have a provision for this but it's usually very small or blocked with what's known as a ‘giggle pin’.

Sometimes it may help leaving the radiator cap off during this process, slightly squeezing the top hose.

Eliminating the problem during installation with the proper equipment makes it easier but expensive for the average DIY. Some workshops use a machine to pump (exchange) the coolant into the system forcing the air out simultaneously. Generally, with this method the system does not need to be topped off or have any air lock problems. Workshops may also draw a vacuum on the system and have the vacuum pull the coolant into the system (atmospheric pressure actually pushes it).


Pump Air Lock

As the description says the air pocket is in the pump impeller cavity. This location is the least common and not likely when using Evans.

On the initial fill the engine will run very hot very quick within a few minutes. This is due to flow being completed restricted; the impeller is spinning in air not liquid.  The air lock can also happen while driving (suction side leak) the temperature gauge will peg than come back to normal than peg again quickly. This is a result of the air in the impeller cavity stopping circulation and then working though with circulation returning. In some cases temperature gauge needle will move so much that it will resemble a tachometer.

In certain pump designs coolant flow may not easily move the air though the pump, causing the air pocket to cavitate the impeller.  Small and Big block Chev and most of the early Ford engines are only a few examples. The remedies mentioned above will lessen the chances of pump air lock.

In most cases the water pump weep hole is the source and will allow air in but not leak coolant. This is why it may be an option to plug the weep hole or install air bleed lines. This is still a viable modification to most pumps, but not easily done once installed.

The current version of Evans has a lower viscosity than early versions so experiencing a pump air lock is highly unlikely.


About Evans Cooling Systems Australasia:

Evans Cooling Systems Australasia is a 100% Australian owned company, based in Melbourne, Victoria, continually seeks to improve engine performance and efficiency, and to contribute to a cleaner and safer environment.  For more information on Evans Cooling Systems Australasia and Evans waterless engine coolants please visit or contact us on