The COBB Tuning Air Oil Separator (AOS) is a precision-engineered component designed to improve engine efficiency and reliability in the 2015–2020 Subaru WRX. By separating oil from the intake system and recirculating it back to the crankcase, the AOS minimizes oil consumption and reduces the risk of carbon buildup in the intake manifold and turbocharger. This guide provides a detailed, step-by-step walkthrough of the installation process based on official documentation from COBB Tuning. The instructions are specific to the 2015–2020 WRX models and emphasize proper routing, securing, and alignment of components to ensure safe and effective operation.
Introduction
The installation of the COBB AOS involves several key steps, including removing the top-mount intercooler, installing the AOS bracket, routing coolant lines, and configuring the Positive Crankcase Ventilation (PCV) system. The process requires a set of specific tools and an understanding of the vehicle’s engine layout to ensure that all components are properly aligned and secured. The AOS also includes a coolant bleeder valve to facilitate air removal after installation, a critical step in maintaining optimal engine performance.
This guide is structured to provide clarity and precision, emphasizing the importance of adhering to recommended torque specifications and avoiding over-tightening of plastic fittings. Users are advised to ensure the engine has cooled down before beginning the installation and to take care when handling coolant to minimize spillage and contamination. Additionally, the installation may require adjustments to existing components such as the strut tower bolts and AC compressor fittings to accommodate the AOS bracket and hose routing.
Tools and Parts Required
Before beginning the installation process, ensure you have the following tools and parts on hand:
Parts Included in the COBB AOS Kit
- Air/Oil Separator (AOS) unit
- (43") Top coolant hose assembly with bleeder valve (1/2" ID -8ORB)
- (15") Bottom coolant hose assembly (1/2" ID -8ORB)
- Block Breather Hose Assembly (44" 1/2" hose with 90° fitting)
- Oil drain and lower breather hose assembly
- Upper recirculation housing and o-ring (preinstalled)
- Recirculation hose assembly (30" 5/8" ID with Y fitting)
- AOS Drain Fitting
- 1/2" Spring Clamp
- Mounting Bracket
- (3) M6x10mm bolt
- 5/8" High temp cap
- 1/2" High temp cap
- (20) 8" Zip tie
- ECU Relocation bracket
- (2) M6x20mm Bolts
- (2) M6 Washers
- 3/8" 90° plastic fitting
- (23") 3/8" PCV Hose
- 3/8" NPT to 5/8" Brass barb fitting
- (2) M6x12mm bolt
Recommended Tools
- Phillips head screwdriver
- Flathead screwdriver
- 3/8" ratchet
- 3/8" 10mm socket
- 3/8" 12mm socket
- 3/8" 19mm socket
- 3/8" 12" extension
- 3/8" 6" extension
- 1/4" Allen socket
- M3 Allen Socket
- M5 Allen Socket
- 10mm combination wrench
- 12mm combination wrench
- 7/8" combination wrench
- 11/16" combination wrench
- Trim removal tool
- Needlenose pliers
- Hose cutter
- Electrical tape
It is important to note that the AOS installation may also require the removal and reinstallation of the top-mount intercooler and charge pipe, as well as minor modifications to existing routing paths for hoses and fittings. Users are encouraged to plan for additional time to accommodate these steps and ensure a smooth installation.
Vehicle Preparation
Prior to beginning the installation, it is essential to prepare the vehicle to ensure a safe and efficient process. Begin by parking the vehicle on a flat, level surface to provide stability during the installation. Next, locate and disconnect the negative terminal of the vehicle’s battery using a 10mm wrench. This step helps prevent accidental electrical shorts during the installation process.
If the intercooler is in place, proceed with its removal. This involves removing the two trim clips from the top corners of the intercooler using a trim removal tool or a flat-blade screwdriver. Carefully lift the engine cover from the back and pull it toward you at the front to gain access to the intercooler. Remove the 8mm hose clamp securing the factory charge pipe to the passenger's side and pull the pipe free. Next, remove the three 12mm bolts that hold the intercooler in place at opposite corners. Once the driver's side intercooler support bracket is removed, gently lift and remove the intercooler from the engine bay.
After the intercooler is removed, the charge pipe can be taken off. This step may require loosening additional clamps and disconnecting other components that are in the way. Once the charge pipe is free, it can be set aside for later reinstallation or kept aside as needed depending on the routing of the AOS coolant lines.
AOS Bracket Installation
With the vehicle properly prepared and the intercooler removed, the next step is to install the AOS bracket. The bracket should be positioned to point downwards and secured to the strut tower using the original M6 bolts. The bracket is typically mounted in the second from the bottom hole to ensure proper clearance for other components such as the bleeder port and brake lines. If the bracket interferes with existing components, minor adjustments may be necessary to accommodate the AOS bracket.
Once the bracket is in place, install the two coolant lines into the bottom of the AOS using a 10mm Allen socket. The lines should be tightened to 8Nm (6 ft/lbs) to ensure a secure connection without over-tightening. The longer coolant line with the included bleeder valve should be connected to the top port of the AOS. It is important to route the coolant lines carefully to avoid kinks or obstructions and to secure them with zip ties to prevent movement and chafing.
The coolant lines must be routed along the path that avoids any sources of heat and ensures that they are free of obstructions. It may be helpful to test fit the charge pipe back in place to confirm that the coolant lines do not interfere with other components. Additionally, the coolant lines should be secured with zip ties at multiple points to prevent movement and ensure a clean installation.
Coolant Line Installation
After the AOS bracket is installed, the next step is to install the coolant lines. Locate the 90-degree bent hose that connects the engine block to the turbocharger. This is the coolant line that will be modified to accommodate the AOS. Begin by clamping the coolant line using an appropriate tool to prevent coolant from leaking during the installation process.
Route the upper coolant line along the desired path, ensuring that it does not interfere with other components in the engine bay. It may be helpful to test fit the charge pipe back in place to confirm that the coolant line does not get in the way. The longer hose from the top of the AOS will connect to the turbo itself. It is important to work quickly during this step to minimize coolant spillage.
Once the coolant line is connected to the turbo, take the original line and rotate it around to the side. Loosen the hose clamp to facilitate this step. Route the line around to the fitting on the lower coolant hose from the AOS and secure it in place. Once the hose clamps are in place at all connection points, the clamp can be removed. It is recommended to double-check the hose coming from the block to ensure that it is not crimped where it turns.
After the coolant lines are installed, it is important to bleed the coolant to remove any air pockets that may have formed during the installation. The upper coolant hose on all COBB AOS units includes a small bleeder valve to facilitate this process. With the engine cold but running, use a wrench to hold the fitting closer to the AOS and another wrench to loosen the outer fitting, allowing coolant and air to escape through the weep hole. Continue this process until no more air bubbles are observed. This step is critical to ensure proper engine cooling and to prevent overheating or incorrect coolant temperature readings.
PCV System Configuration
The Positive Crankcase Ventilation (PCV) system is an essential component of the AOS installation, as it ensures proper recirculation of crankcase gases and prevents the buildup of oil vapor in the intake system. The PCV system for the 2015–2020 Subaru WRX requires specific routing and installation steps to ensure optimal performance.
Begin by removing the white blow-by sensor from the single hose side of the large Y-pipe. Secure the connection using the provided spring clamp to ensure a tight fit. Install the double-sided end of the Y-hoses to the top of the AOS using spring clamps to hold them in place. Route the other end of the Y-hoses around and down to the inlet, securing it with a spring clamp or by reinstalling the sensor onto the inlet with the new hose fitted.
Take the remaining hose and install it over the fitting behind the AC compressor using a spring clamp. Route it to the final remaining port (upper side port) and secure it with a hose clamp. For a cleaner appearance, it is recommended to route the hose under the intake manifold to minimize clutter in the engine bay.
On 2018+ models, additional steps may be required due to the presence of a blow-by sensor. In these models, it is necessary to undo the electrical connection and remove the sensor assembly from the inlet with the hose. Removing the hose clamp from the side with the AC is sometimes easier by cutting the rubber out from under the permanent metal ring and removing the ring after the hose is out. Once the hose is removed, the sensor can be reinstalled onto the inlet with the new hose fitted.
The PCV valve should be installed with the arrow pointing towards the engine to ensure proper flow direction. The PCV line should be routed through the PCV housing port by the COBB logo and secured with a spring clamp. It is important to ensure that the PCV system is properly sealed and free of leaks to maintain optimal engine performance.
Final Installation and Testing
Once all components are installed and secured, it is important to double-check the installation for any potential issues. Ensure that all hose clamps are properly tightened, and that the coolant lines are routed away from sources of heat to prevent damage or melting. Check that the AOS bracket is securely mounted and that there is enough clearance for the bleeder port and other components.
After the installation is complete, it is time to bleed the coolant system to remove any air pockets that may have formed during the installation. This step is critical to ensure proper engine cooling and to prevent overheating or incorrect coolant temperature readings. The upper coolant hose on all COBB AOS units includes a small bleeder valve to facilitate this process. With the engine cold but running, use a wrench to hold the fitting closer to the AOS and another wrench to loosen the outer fitting, allowing coolant and air to escape through the weep hole. Continue this process until no more air bubbles are observed.
Once the coolant system is properly bled, it is important to recheck all connections for leaks and ensure that the engine is running smoothly. If any issues are detected, such as coolant leaks or unusual engine behavior, it is recommended to consult a qualified and experienced automotive technician for further assistance.
Conclusion
The installation of the COBB Tuning Air Oil Separator on the 2015–2020 Subaru WRX is a complex but manageable process that requires attention to detail and adherence to recommended procedures. By following the steps outlined in this guide, users can ensure a safe and effective installation that improves engine efficiency and reliability. The AOS helps reduce oil consumption and minimizes the risk of carbon buildup in the intake manifold and turbocharger, making it a valuable addition to any performance-oriented WRX setup.
Users are encouraged to carefully read through the installation instructions before beginning the process to familiarize themselves with the steps and tools required. If there is any uncertainty about the installation process, it is highly recommended to consult a qualified and experienced automotive technician to ensure proper installation and avoid potential damage to the vehicle.