Your Guide to Subaru Air Oil Separator Replacement

Your Subaru’s Boxer engine is a finely tuned machine, but it has an Achilles' heel: oil blow-by. This issue allows oil into the combustion chamber, where it doesn't belong, increasing the risk of catastrophic engine knock. An Air Oil Separator is the most effective insurance policy against this threat. By installing a quality AOS, you are actively preventing the root cause of many engine failures. It’s a relatively small investment that provides immense peace of mind by safeguarding your pistons, rods, and bearings from damage. This guide will walk you through why a Subaru air oil separator replacement is a non-negotiable part for engine longevity and reliable performance.

Key Takeaways

• An AOS is a protective necessity, not just an upgrade: It prevents oil vapor from contaminating your intake, which maintains your fuel's octane rating and stops destructive engine knock. This is especially critical for any modified or turbocharged Subaru.
• Recognize the warning signs of a failing system: Pay attention to increased oil consumption, blue smoke from the exhaust, and sluggish performance. Identifying these symptoms early is key to preventing more serious and costly engine damage.
• Invest in a maintenance-free, closed-loop system: A high-quality AOS automatically separates and returns oil to the engine, making it a "set it and forget it" solution. This is a far more effective and reliable choice than a basic catch can that requires constant manual draining.

What is a Subaru Air Oil Separator?

Think of an Air Oil Separator (AOS) as your engine's personal bodyguard. In any engine, but especially in high-performance Subaru Boxers, pressure builds up in the crankcase. This pressure, a mix of air, unburnt fuel, and oil vapor, has to go somewhere. A standard PCV (Positive Crankcase Ventilation) system vents these gases back into your intake manifold to be burned off. The problem? That oil vapor comes along for the ride. When oil contaminates your air-fuel mixture, it lowers your fuel's octane rating and can lead to a host of performance issues, from reduced power to serious engine knock.

An AOS is an elegant solution that intercepts these vapors before they reach your intake. It’s designed to separate the oil from the air, sending clean air back to the intake and draining the collected oil safely back into your engine's oil pan. Unlike a simple catch can that just collects the gunk and needs to be emptied regularly, a quality Subaru Air Oil Separator is a true closed-loop, maintenance-free system. It’s a must-have for any modified Subaru, providing a critical layer of protection whether you're on the track, trail, or just enjoying a spirited daily drive. By keeping your intake clean, an AOS ensures your engine performs consistently and reliably, protecting your investment in the long run.

How an AOS Works in Your Subaru Engine

The magic of an AOS happens inside its canister. As the crankcase vapors are routed into the unit, they pass through a series of baffles or a centrifugal chamber. This process forces the heavier oil and fuel droplets to separate from the lighter air. Gravity takes over, and the collected oil drains down to the bottom of the AOS. From there, it’s returned directly to the oil pan, keeping your oil levels consistent. Meanwhile, the now-clean air is sent back into the engine’s intake tract to be burned as intended. This continuous, self-contained cycle ensures your engine is only breathing in fresh air, not a performance-robbing oil mist. It’s a simple yet highly effective process that makes the AOS one of the most important supporting performance parts you can install.

How an AOS Prevents Engine Damage

By keeping oil out of your intake, an AOS directly prevents one of the biggest threats to a tuned engine: detonation, also known as engine knock. When oil vapor mixes with the air and fuel entering your combustion chamber, it effectively lowers the octane rating of your fuel. This makes the mixture more prone to igniting prematurely under pressure, creating a destructive shockwave inside the cylinder. This is especially dangerous in turbocharged engines like those in the WRX and STI. An AOS ensures a clean, potent air-fuel charge, which allows your engine to run safely at its peak potential. It protects your pistons, rods, and bearings from the catastrophic damage that detonation can cause. In short, it’s not just a performance part—it’s essential insurance for your engine’s health and longevity.

Signs Your Subaru AOS is Failing

A failing Air Oil Separator doesn't usually announce itself with a loud bang. Instead, it sends subtle signals that can be easy to miss if you aren't paying attention. This component works quietly in the background, but when it starts to fail, the consequences can be significant for your engine's health and performance. Ignoring the early signs allows oil to contaminate your intake system, which can lead to reduced performance and, eventually, serious engine damage. Catching the problem early is key to protecting your investment and keeping your Subaru running at its peak, whether you're on the street, track, or trail. Think of it as preventative care for your engine's heart. If you notice any of the following symptoms, it’s time to take a closer look at your AOS. These signs can range from visible smoke and higher oil consumption to a gradual loss of power, and identifying them is the first step toward a solution that keeps your engine healthy and your performance consistent. Don't wait for a check engine light; being proactive here can save you from a much larger repair bill later on.

Oil in the Intake & Excess Smoke

One of the most telling signs of a failing AOS is finding oil in your intake tract, intercooler, or turbo inlet. The entire job of the AOS is to prevent this from happening. When oil vapor gets past the separator and into the intake, it lowers the octane rating of your fuel-air mixture. This can cause engine "knock," also known as detonation, which can severely damage your pistons and reduce power. You might also see puffs of blue-ish smoke from your exhaust, especially during hard acceleration, as this oil is burned off in the combustion chamber. This isn't just messy; it's a direct symptom of a system that's no longer protecting your engine's most critical performance parts.

Poor Performance & High Oil Consumption

Is your Subaru feeling a bit sluggish or less responsive than it used to be? A failing AOS could be the culprit. When oil coats the inside of your intercooler, it reduces its ability to cool the air entering your engine, leading to a drop in performance. You may also find yourself adding oil between changes more often than usual. This increased oil consumption happens because the oil that should be returned to the crankcase is instead being sent into the intake and burned away. Neglecting these symptoms can lead to more significant issues down the road, making a high-quality AOS a critical component for maintaining engine health and getting the most out of your engine tuning services.

How to Test Your AOS with a Manometer

If you suspect your AOS is failing but want a definitive answer, a diagnostic test is your best bet. The most reliable way to check if your AOS is working correctly is to use a tool called a manometer to measure your engine's crankcase vacuum. This simple test involves connecting the manometer to your oil dipstick tube and reading the pressure at idle. A healthy PCV system, managed by the AOS, should pull a slight vacuum. If you see positive pressure, it’s a clear sign that the system isn't functioning properly and crankcase vapors aren't being evacuated effectively. This is a straightforward test you can do at home or ask your trusted mechanic to perform.

Benefits of an Upgraded AOS

Swapping your stock Air Oil Separator for a high-performance model is one of the smartest investments you can make for your Subaru’s engine. This isn't just about replacing a part; it's a proactive upgrade that delivers tangible benefits in engine protection, efficiency, and overall performance. A quality AOS acts as a guardian for your engine, ensuring that only clean air enters your intake system. This is especially critical for turbocharged engines, where maintaining a clean air charge is essential for consistent power and long-term health. Whether you’re building a daily driver, a track weapon, or an off-road rig, an upgraded AOS addresses the inherent weaknesses of the factory PCV system. By effectively removing oil and other contaminants from the crankcase vapor, it prevents a cascade of issues that can rob your engine of power and eventually lead to costly damage.

Protect Your Engine & Reduce Carbon Buildup

The primary job of an upgraded AOS is to protect your engine from the inside out. Your Subaru’s engine naturally produces crankcase pressure that contains oil vapor. A high-performance AOS intercepts this vapor, separating the oil and returning it to the crankcase while allowing clean air to pass into the intake manifold. This process is crucial for preventing carbon buildup on your intake valves and pistons. Over time, that buildup can restrict airflow and create hot spots in the combustion chamber, leading to detonation, or "engine knock," which can cause catastrophic engine failure. By installing an upgraded AOS, you ensure your engine breathes cleaner air, which helps maintain its power and significantly extends its lifespan.

Stop Oil Blow-By & Improve Intercooler Efficiency

Oil blow-by is a common issue in performance engines, where oil vapor coats the inside of your intake tract and intercooler. This oily residue acts as an insulator, drastically reducing your intercooler's ability to cool the charged air before it enters the engine. Hotter air is less dense, which means less power and a higher risk of detonation. An upgraded AOS stops this process at the source. By capturing that oil before it can contaminate your system, it keeps your intercooler working at peak efficiency. This ensures you get the consistent, reliable power you expect, making your engine tuning services more effective and your vehicle more responsive.

Improve Track & Off-Road Performance

When you’re pushing your Subaru on the track or navigating a tough trail, your engine is under extreme stress. High G-forces, aggressive cornering, and steep inclines cause oil to slosh around, overwhelming the factory PCV system and leading to increased oil consumption. This is exactly when you need your engine to perform flawlessly. A purpose-built AOS is designed to handle these demanding conditions, ensuring a steady supply of oil-free air to the engine, no matter the situation. This upgrade provides the reliability and consistent performance needed for lap-after-lap track days and is a vital component for any serious overlanding solutions build where engine dependability is non-negotiable.

How to Choose the Right AOS for Your Subaru

With several excellent Air Oil Separators on the market, picking the right one comes down to your specific Subaru model, your driving style, and the features that matter most to you. Each brand brings something different to the table, whether it's a track-tested design or specific features for daily driving. Understanding these differences is key to making an informed decision that will protect your engine for years to come. The goal is to find a system that not only fits your vehicle perfectly but also aligns with how you use it, from daily commutes to demanding track days or off-road adventures.

When you’re ready to upgrade, consider the materials, design, and reputation of the manufacturer. A well-engineered AOS is a critical investment in your Subaru’s health and performance, preventing costly issues down the road. Let’s break down some of the top contenders to help you find the perfect match for your build.

Crawford Performance AOS Systems

As the original and proven solution for Subaru engines, the Crawford Performance AOS systems are engineered with one primary goal: to protect your engine and extend its life. These systems are meticulously designed to prevent oil blow-by from contaminating your intake and intercooler. This is essential for maintaining consistent performance and preventing detonation, a phenomenon that can cause serious engine damage. By effectively separating oil vapor from the crankcase gases, our AOS ensures only clean air is recirculated, keeping your engine running strong whether you're on the street or the track.

PERRIN vs. IAG Performance

When exploring other options, you'll likely come across PERRIN and IAG Performance, both of which are respected names in the Subaru community. The PERRIN Air Oil Separator is designed to reduce carbon buildup and minimize detonation risk, featuring a heated base to prevent oil sludge from forming in colder conditions. IAG Performance also offers a wide variety of AOS kits tailored to different Subaru models, including a "Universal Drag Catch Can" for dedicated racing applications. Both brands focus on engine protection, but their designs and features cater to slightly different priorities and use cases.

Key Features & Model Compatibility

When making your final choice, focus on two things: key features and model compatibility. Do you need a heated unit for a cold climate, or is a track-proven, maintenance-free system like Crawford’s a better fit? Most importantly, you must confirm the AOS is designed for your specific Subaru. An AOS for a WRX won't necessarily fit a Crosstrek. Always check the product specifications to ensure it matches your vehicle’s model and year range. This guarantees proper installation, optimal function, and the engine protection you’re looking for.

AOS Installation: What to Expect & How Much It Costs

So, you’re sold on the benefits of an AOS and ready to get one installed. The next logical questions are: what does the installation involve, how long will it take, and what’s the damage to your wallet? Getting this performance part on your Subaru is usually a straightforward affair, but the details matter. Whether you're a seasoned DIYer ready to spend a Saturday in the garage or you prefer to hand the keys to a trusted mechanic, understanding the process and costs will help you make the right call. This section breaks down the installation timeline, the price range for quality units, and the pros and cons of installing it yourself versus having a professional handle it.

The Installation Process & Timeline

Installing an Air Oil Separator is a project you can likely tackle in an afternoon. Most installations take around three hours, making it a manageable weekend task for anyone comfortable working on their car. The process typically involves mounting the AOS unit securely in the engine bay and then routing hoses to connect it to your engine's crankcase ventilation system. While the exact steps vary depending on your specific Subaru model and the AOS kit you choose, the goal is always the same: to intercept the oily vapor before it reaches your intake and causes performance issues.

Cost Breakdown: Brand & Quality

The price for an Air Oil Separator can vary quite a bit, so it’s important to know what you’re paying for. You can find basic units for under $150, but high-performance systems from reputable brands typically range from $400 to $500. For example, our own Crawford Performance AOS kits are engineered for maximum efficiency and durability. The price difference often comes down to design complexity, material quality, and the brand’s engineering and testing. A well-made AOS is an investment in your engine’s health, so choosing a quality unit is key to getting the protection you need.

DIY vs. Professional Installation

Deciding whether to install the AOS yourself or hire a pro comes down to your comfort level and toolset. If you have basic tools and enjoy working on your car, this is a project you can definitely do yourself. However, if you’re not confident, there’s no shame in leaving it to the experts. Think of it this way: a $400 part and a few hours of labor can prevent a potential multi-thousand-dollar engine rebuild down the road. For that kind of peace of mind, professional engine tuning services are a smart investment to ensure everything is installed and functioning perfectly.

Where to Buy Your Subaru AOS

Once you’ve decided to upgrade, the next step is finding the right place to purchase your Air Oil Separator. You want to buy from a source that not only offers a quality product but also provides solid support and stands behind their parts. Investing in a critical component like an AOS from a reputable seller ensures your engine gets the protection it deserves, giving you peace of mind whether you're on a daily drive or pushing your car to its limits.

Trusted Retailers & Online Stores

When shopping for an AOS, it’s best to start with specialists who live and breathe Subaru performance. As the creators of the original Subaru AOS, we at Crawford Performance offer a range of proven Air Oil Separators designed for nearly every model from 2002 to the present. Other well-known brands like IAG Performance and PERRIN also offer their own systems through various online retailers. While marketplaces like eBay can show you a wide selection of parts, sticking with a dedicated performance shop or the manufacturer directly often gives you better access to expert advice and support for your specific build.

Promotions & Warranty Information

Getting a good deal is always a plus, but the real value comes from a product’s long-term reliability. Some retailers may offer price matching or seasonal promotions, but a strong warranty is far more important. A warranty is a manufacturer's promise to stand by their product's quality and engineering. At Crawford Performance, we back our performance parts with a unique 12-month/12,000-mile engine warranty, giving you confidence that your investment is protected both on the track and on the trail. Always check the warranty details before you buy, as it speaks volumes about the manufacturer's trust in their own product.

Post-Installation Maintenance

One of the best things about a modern, well-designed AOS is that it’s largely a "set it and forget it" modification. Unlike old-school catch cans that require you to manually drain collected oil, a high-quality AOS automatically separates the oil and returns it to the engine crankcase. This closed-loop design means there’s no regular emptying or cleaning required on your part. The system works continuously in the background to protect your engine, keeping your intake clean and your intercooler efficient without adding another maintenance task to your list. This simplicity makes it one of the most effective and hassle-free upgrades you can make for your Subaru.

Related Articles

• All About Subaru's Original Air Oil Separator & Why You Need One
• 4 Best Air Oil Separators for WRX (Ranked)
• 2008-2014 Subaru WRX Air Oil Separator V2 Crawford Performance

Frequently Asked Questions

Is an AOS only for heavily modified Subarus? Not at all. While an Air Oil Separator is essential for tuned and high-performance builds, even a completely stock Subaru can benefit. The direct-injection engines in newer models are particularly prone to carbon buildup on the intake valves, which an AOS helps prevent. Think of it as a preventative health measure for your engine, ensuring its longevity and efficiency regardless of your power level.

What's the real difference between an Air Oil Separator and a catch can? The main difference is that an AOS is a complete, closed-loop system, while a catch can is a simpler, open-loop solution. A catch can does just what its name implies—it "catches" the oil and gunk in a container that you have to manually empty on a regular basis. An AOS, on the other hand, separates the oil and automatically drains it back into your engine's oil pan, making it a true set-it-and-forget-it upgrade with no maintenance required.

Will installing an aftermarket AOS void my car's warranty? This is a common concern, but installing an AOS should not automatically void your vehicle's warranty. Legally, a manufacturer has to prove that the aftermarket part you installed was the direct cause of a specific failure. Since an AOS is designed to protect your engine by improving on the factory's ventilation system, it's difficult to argue that it would cause damage. However, it's always a good idea to speak with your dealership's service department if you have concerns.

Do I need to get my car retuned after installing an AOS? Generally, no. An AOS is considered a reliability modification, not a performance part that alters your engine's air-fuel parameters. It simply ensures your engine is breathing clean, oil-free air, which allows your existing tune to perform as it was intended. It corrects a weakness in the factory system rather than changing how the engine is meant to run, so a retune is not necessary.

Are there any special considerations for using an AOS in cold climates? Yes, this is an important factor. In freezing temperatures, condensation can mix with the oil vapor inside the AOS and create a thick sludge that can block the drain-back line. To prevent this, some AOS systems are designed to be heated by your engine's coolant. This keeps the unit warm enough to ensure the separated oil stays fluid and drains properly back into the engine, providing reliable protection year-round.