What Does an Air Oil Separator Do? (All Explained)

Written by

Charles Bolte


Vernon Hoppe 

what does an air oil separator do

The internal combustion engine is the beating heart of your car. As fuel injectors push air to your combustion chamber, it causes compression along with fuel that transfers energy to your vehicle.

During this process, engine oil is injected, lubricating and absorbing the excess heat. The air-oil separator is a filter that separates the oil from the air.

While clean compressed air is removed from the compressor, the oil is returned to the compressor for lubrication. It plays an essential role in maintaining the health and performance of your car’s engine.

In this article, we will take a deep dive into: What does an air oil separator do, its benefits, and do you really need it?

How Does an Air Oil Separator Work?

While the engineering behind air-oil separators seems complex, the working principle is quite simple.

  1. The air and oil mixture is passed through a tube to the air and engine oil separator.
  2.  The air flows out of the filter at a hard angle and the much denser oil stays inside the low-pressure filter. The centrifugal force caused by the motion filters about 85% of the oil.
  3. The remaining oil with compressed air are tiny droplets caught by the separator filter and collected at the bottom.
  4. The air and oil separator then returns the clean oil back to the compression chamber.
Credit: Radiumauto.com

Check out this air/oil separator diagram from the air oil separator for WRX Subaru engines for a clearer understanding of the process.

Air Oil Separator Benefits

Now that we know how an air oil separator functions, let’s take a look at how this device can be a great addition to your car:

1. Prevent oil build-up


The oil that returns to your engine’s cylinders can form carbon deposits, accumulate in the air intake, and cause a clog and stifle airflow, leading to rough idling and engine misfires. Air and lube oil separators avoid this problem by only allowing filtered air to return to the intake.

2. Prevent premature combustion


Oil is a very volatile substance and can create small explosions in your car. If oil is unregulated due to having no air oil separators, combustible oil can cause premature combustions on delicate parts of the engine and can cause a lot of damage.

3. Minimize oil loss


Since air and oil separators keep the oil inside the enclosed car system and return it to the compressor, there is a minimal loss of oil in the process. This allows your car to maintain a high standard of lubrication and can keep it operational for a long time.

Different Types of Air Oil Separators


Air-oil separators come in many shapes and sizes for different compressor and engine types. The two major types of oil separators that are favorable in most engines are cartridge and spin-on for their versatility.

  • Cartridge – This separator system has a cartridge that can be replaced when no longer usable. Thus, making it convenient to alter the old one after turns of filtering oil mists.
  • Spin-On – This separator has a threaded end that can fit for your screw compressor and can be easily removed and replaced as needed.

Do I Really Need an AOS?


Whether your car needs or requires an air-oil separator is dependent on many factors, including the type of vehicle, driving conditions, and personal preference. In this section, we will discuss why an AOS might be necessary for your vehicle.

  • Performance-Focused

Filtered oil means that the engine is cleaner and will run more efficiently. Lack of contamination also means that the car will have better mileage. This is perfect for car owners that want to focus their cars on performance.

  • Direct Injection Engines

Direct injection engines force the car’s fuel directly into the combustion system. This gives them better performance and fuel economy.

Installing an air-oil separator is important for these types of engines since it reduces the risk of contamination from oil vapors and residue that can be residuals from internal combustion.

  • Reduce Engine Clog

Oil build-up can cause used oil to return to the combustion cylinders. This can cause clogging which stifles airflow and decreases performance. Blocked cylinders reduce combustion inside your car and using an AOS can help if this is a recurring problem.


Catch can vs air oil separator: are they the same?

Just like the air-oil separator, a catch can capture the excess oil after combustion. However, it does not recirculate the oil into the engine. This requires you to constantly check and maintain the level of oil in the catch can.

How much does it cost to install an oil separator?

This is dependent on the make, model, and brand of oil separators. IAG air oil separators, for example, can cost between $373.99 – $429.99. Labor costs can also be added to this initial charge if you choose to have a reputable service center do it for you.

Maintenance and care of an Air Oil Separator

Air oil separators should be cleaned and maintained after 1000 hours of use. A non-corrosive cleaning liquid should be used to clean the filter and its housing. Also, make sure that the valve and strainer are properly fitted into the device after cleaning.


Air oil separators are optional devices that can be added to an engine to improve fuel efficiency and reduce contamination in your car’s combustion system.

This very simple renovation offers a ton of benefits and can make your car run for a good long time. Remember these the next time someone asks you the question: What does an air oil separator do?

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