IAG AIR / Oil Separator (AOS) - 2002-2007 Subaru WRX/STI & 2004-2008 Forester XT

Essential Protection for the Legendary EJ Platform
On turbocharged EJ-series engines (EJ205, EJ255, and EJ257), aggressive driving and elevated boost levels generate high crankcase pressures that blow oil mist and vapors directly into your intake tract. Once inside, this oil coats the interior walls of your top-mount or front-mount intercooler, drastically reducing its ability to cool the intake air charge. Even worse, when that oil enters the combustion chamber, it dilutes your fuel, lowers your effective octane rating, and induces dangerous engine knock. The IAG V3 Air Oil Separator (AOS) is engineered specifically to capture and separate this oil before it ever reaches your intake, preserving horsepower, intercooler performance, and engine longevity.

Patent-Pending Swirl-Pot Design with Integrated Thermal Heating
Unlike cheap catch cans that use simple wire mesh baffles and require constant manual emptying, the IAG V3 AOS is a permanent, maintenance-free solution. Its high-volume cast aluminum housing utilizes a multi-stage swirl pot layout that spins oil out of the crankcase air stream using centrifugal force. The separated oil automatically drains safely back down into your oil pan. To prevent cold-weather moisture buildup and the dreaded "milky sludge" common in northern climates, IAG routes engine coolant directly through the base of the separator body. This keeps the unit at engine operating temperature, completely eliminating condensation and freezing risks during winter driving.

• Stop Oil Contamination in Intake & Intercooler: Keep oil mist out of your intake, keeping intercooler cores clean and operating at maximum thermal efficiency.
• Prevent Engine Knock & Detonation: Stop crankcase oil vapors from entering the combustion chamber where they lower effective fuel octane and trigger knock retard.
• Heated Housing Technology: Taps into your factory cooling system to keep the AOS warm, preventing water condensation, sludge buildup, and freezing in cold climates.
• Maintenance-Free Drain-Back System: Automatically returns captured oil directly to the oil pan. No messy catch cans to manually drain at every gas fill-up.
• GD & SG Chassis-Specific Fitment: Includes vehicle-specific mounting brackets, custom-molded hoses, and fittings designed exclusively for the 2002–2007 WRX/STI and 2004–2008 FXT engine bays.

Technical Specifications:

• Product Type: Air Oil Separator (AOS)
• Housing Material: Heavy-Duty Cast Aluminum
• Separation Method: Multi-Stage Swirl Pot & Baffling
• Thermal Control: Integrated Coolant Passage (Heated Base)
• Drain-Back Style: Automatic Gravity Drain to Engine Oil Pan
• Hose Construction: Custom Molded SAE-Rated PCV & Breather Hoses
• Engine Compatibility: 2.0L & 2.5L Turbo EJ Boxer Engines (EJ205 / EJ255 / EJ257)
• Maintenance Requirement: Zero (Self-Draining System)

Fitment:

• Subaru Impreza WRX: 2002, 2003, 2004, 2005, 2006, 2007 (2.0L EJ205 & 2.5L EJ255 Turbo)
• Subaru Impreza WRX STI: 2004, 2005, 2006, 2007 (2.5L EJ257 Turbo)
• Subaru Forester XT: 2004, 2005, 2006, 2007, 2008 (2.5L EJ255 Turbo)
• Saab 9-2X Aero: 2005, 2006 (2.0L EJ205 & 2.5L EJ255 Turbo)

The Nameless "Installation" Pro-Tips:

Coolant Bleeding & Hose Slope Are Everything:

1. Respect the Gravity Drain: The bottom oil return line depends entirely on gravity to feed separated oil back into the crankcase. Make sure the hose maintains a smooth, continuous downward slope from the AOS body down to the engine block with zero sags or "U" dips. Any low spot will trap liquid oil, preventing proper drainage and backing up the separator.

2. Mind Your EJ Breather Routing: EJ engines have distinct crankcase PCV ports and dual valve-cover breather ports. Take your time following IAG's instructions to ensure you connect the head breathers and crankcase lines to their corresponding AOS ports. Proper routing balances crankcase pressure across all four cylinders under heavy boost.

3. Burp the Cooling System: Because you will be splicing into your heater core lines to route coolant through the base of the AOS, air bubbles will get into the cooling system. Always finish the job by thoroughly burping the cooling system using a no-spill funnel to avoid air pockets and erratic engine temperatures!