WRX Oil Catch Can Routing: Do It Right the First Time

A catch can only works as well as the lines connected to it. On a WRX, the correct routing depends on whether you are working with an EJ or FA engine. Which crankcase vapors you need to intercept, and how the system handles boost and vacuum.

That makes routing more than a matter of placing a can in the easiest open space. First, it helps to understand what the PCV and breather paths are doing, then the EJ and FA differences become much easier to follow.

Contact Crawford Performance to confirm the right routing for your WRX.

What Is WRX Oil Catch Can Routing?

WRX oil catch can routing is the planned path for crankcase vapors to leave the engine. Pass through a separator, and return to the intake without carrying as much suspended oil. The goal is not simply to place a can somewhere in the engine bay. The hoses, ports, valve orientation, and flow direction must preserve the factory crankcase ventilation strategy.

During combustion, a small amount of pressure and combustion gas can move past the piston rings into the crankcase. This is commonly called blow-by. The piston ring pack is also a significant area of oil vaporization and oil-vapor transport in an internal combustion engine, as documented by MIT research: MIT's piston ring pack study. Those vapors can contain oil mist and other contaminants that the ventilation system carries away from the crankcase.

In a typical routing arrangement, the catch can inlet connects to a crankcase or PCV vapor source. The outlet then connects toward the intake, allowing the engine to maintain ventilation while the can separates part of the oil from the vapor stream. In simple terms, in comes from the crankcase, and out goes to the intake. Reversing those connections can undermine separation and disrupt the pressure balance the PCV system depends on.

Why the ventilation path matters

Modern turbocharged engines can send oily vapor back through the intake repeatedly. On direct-injection engines, fuel does not wash across the intake valves, so oily vapor and combustion byproducts can accumulate there. The University of Illinois Chicago explains why an air-oil separator or catch can is important in that environment: UIC's explanation of intake-valve deposits. Correct routing helps address that source without blocking the ventilation path or creating an unnecessary pressure drop.

The exact connection points vary between EJ and FA engines, so a universal hose diagram is not enough. Older EJ applications commonly separate PCV and breather paths, while FA applications use a differently integrated system that requires closer attention to factory flow. For broader system context, see the Subaru crankcase ventilation system and our Subaru oil catch can guide.

A catch can also creates a maintenance responsibility: separated oil and contaminants collect in the can and must be drained. If you are deciding between a manually serviced can and an oil separator that returns separated oil to the crankcase. The correct choice depends on your engine, use case, and tolerance for maintenance.

Contact Crawford Performance to confirm the right routing for your WRX.

WRX Oil Catch Can Routing for EJ vs FA Engines

The engine family changes which crankcase-vapor paths need attention. EJ and FA engines both use PCV and breather plumbing to move blow-by gases away from the crankcase, but the lines are not arranged the same way. Correct wrx oil catch can routing preserves that intended flow while giving the can a chance to separate oil mist before vapor returns to the intake.

EJ engines: plan around the PCV and breather circuits

On EJ-series WRX and STI models through roughly 2014, the PCV connection and valve-cover breather lines handle separate parts of the ventilation strategy. The PCV circuit serves one side of the system, while breather lines from both valve covers join and route toward the intake.

That layout is why a typical EJ installation often uses a dual catch can setup. One can is assigned to the PCV path, and the other is used to intercept the combined crankcase breather path. The goal is to address both sources of oil-laden vapor instead of placing one can in only one branch and assuming the entire system is covered. Crawford's Subaru crankcase ventilation system guide provides useful background on how those paths work together.

• Identify the PCV line and the breather lines before disconnecting anything.
• Match each catch can inlet to the crankcase-side source and each outlet to the original intake-side destination.
• Keep the two circuits separate unless the kit's instructions specifically call for a shared connection.

FA engines: respect the integrated PCV design

FA-series engines, including the FA20DIT in the 2015 and newer WRX generation and later FA applications such as the FA24, integrate the PCV system differently from the EJ. The system includes PCV plumbing and a larger crankcase breather path, so a catch can cannot be routed by simply copying an EJ installation.

A typical FA kit may use a dual-can arrangement for street use, while some setups focus on the crankcase breather path. The correct choice depends on the kit and engine configuration. The important point is to follow the intended flow through the FA system and avoid adding restrictive fittings. Closing off a branch, or connecting lines in a way that changes factory vacuum and boost behavior. FA routing needs to be deliberate because an incorrect connection can disrupt the ventilation flow the engine was designed to maintain.

Before starting either installation, confirm the engine code, trace every hose to its source and destination, and use the kit's routing diagram. If your WRX has modified plumbing or you are unsure which configuration applies, contact Crawford Performance before cutting or permanently relocating a line.

Which PCV and Breather Lines Should You Intercept?

The correct line depends on where the engine sends crankcase vapor during vacuum and boost. On a WRX, start by identifying the factory PCV line and the larger breather lines connected to the valve covers or crankcase. Do not choose a hose simply because it is easy to reach. A catch can only works when it is placed in the vapor path you intend to clean.

PCV and breather lines do different jobs

The PCV line generally carries crankcase vapor toward the intake manifold when the engine is under vacuum. This is the line most directly associated with the positive crankcase ventilation valve. The breather lines, by contrast, connect the crankcase or valve covers to the intake tract and help relieve pressure when operating conditions change. Depending on the engine and kit, both paths may need filtration or separation.

On an EJ engine, a typical setup addresses the PCV line and the breather lines from both valve covers. Those breather lines may join at a factory T before reaching the intake. A dual-can arrangement is often used so the PCV path and crankcase breather path are handled separately. FA engines use a different factory layout, including a larger crankcase breather connection, so follow a vehicle-specific routing diagram rather than copying an EJ installation.

Use the catch can's flow direction as your map

On most catch cans, in means the hose arriving from the crankcase, PCV valve, or valve-cover breather. Out means the cleaned vapor leaving the can and continuing to the intake or manifold connection. Trace the hose from its engine-side source before cutting anything, then connect that source to the can's inlet. Reversing the ports can reduce separation and may interfere with the intended PCV flow.

Keep the fittings and hose diameter close to the factory size. Overly restrictive adapters, sharp reductions, excessive hose length, or unnecessary bends create pressure drop, which can hinder crankcase ventilation efficiency. Crawford identifies restrictive fittings and high-pressure-drop routing as common installation mistakes. For more background on maintaining your Subaru PCV system, review how the factory paths respond to vacuum and boost before finalizing your layout.

Finally, verify every connection with the engine off, confirm the check valve points in the intended direction, and inspect for kinks after securing the hoses. If the routing diagram for your exact year or engine does not match the physical hose layout, pause and resolve that difference before driving.

Catch Can Drain Placement and Mounting Tips

A catch can only helps if you can service it without making a mess. Mount the can upright, secure it against vibration, and leave enough room below the outlet or drain port to remove the collected fluid. The drain should sit lower than the point where oil leaves the crankcase or ventilation line. So oil and condensed contaminants can settle in the can instead of pooling in the hose. Keep the hose runs smooth and reasonably short. Overly restrictive fittings or unnecessary bends can create pressure drop and hinder crankcase ventilation.

Put the drain where service is practical

Before tightening the brackets, check that the drain valve, plug, or lower bowl can be reached with the car on the ground. A can hidden behind a heat shield or tucked beneath other components may look tidy, but it is easy to ignore at service time. If the can has a dedicated drain, route or position it so the fluid can be collected cleanly without disconnecting the entire system. If it has no drain, plan a regular inspection interval and remove the can for emptying before it becomes full.

Do not treat the first drain as the final maintenance schedule. The amount collected varies with engine condition, driving conditions, boost use, and the specific ventilation path being intercepted. Inspect the can early, record what you find, and adjust the interval from there. A full can can reduce the available separation volume and send captured oil back into the hose system.

Baffled and unbaffled catch cans

An unbaffled can is a simple container that gives vapor a larger, slower space in which to drop suspended oil. It can work, but separation is more dependent on mounting position, temperature, flow rate, and the length of time the vapor remains inside. A baffled can adds internal plates, mesh, or changes in direction that force the vapor to deflect and slow down. That helps oil droplets coalesce and settle, reducing oil carryover when the system is flowing heavily. The basic principle is consistent with established separator designs that use pressure reduction, vortex action, deflection, and flow-path reversal to deposit contaminants at the bottom of a housing.

Choose the can and fittings as a system. A sophisticated internal baffle cannot compensate for a restricted inlet, an incorrectly routed hose, or a drain that cannot be serviced. Verify the inlet and outlet markings, keep the can upright, and make sure the mounting hardware cannot contact a belt, pulley, exhaust component, or sharp edge.

Maintenance versus automatic oil return

Catch cans require manual drainage of accumulated oil and contaminants. A Crawford air-oil separator is designed to separate that oil and return it to the crankcase, avoiding the repeated drain-and-empty routine. That makes a catch can a hands-on option for owners who are willing to inspect and service it. While an AOS offers a more set-and-forget approach when reducing maintenance burden matters.

Common Check Valve Mistakes That Break Your Catch Can Setup

Check valves control the direction of crankcase vapor as engine pressure changes. In a turbocharged WRX, that direction changes between vacuum and boost, so a valve that is missing, backward, or too restrictive can undermine otherwise careful wrx oil catch can routing. Check the following points before finalizing your hoses.

1. Mistake: Leaving out the check valve

   Without a one-way valve, boost pressure can travel into the crankcase instead of staying on the intended intake path. That pressure can force oil past seals or even pop the dipstick from its tube. It can also allow unwanted backflow through the catch can. Fix: Install the check valve on the intake or manifold side where the routing requires one, and confirm that it prevents reverse flow under boost. The valve should support the intended path from crankcase vapor, through the separator, and toward the intake.

2. Mistake: Installing the valve backward

   A check valve may look correctly installed while its internal flow direction is reversed. The result is a restricted or blocked PCV path during normal vacuum operation, followed by incorrect flow when the engine enters boost. Fix: Verify the arrow or marked inlet and outlet before connecting the hose. The inlet should receive vapor from the crankcase side, while the outlet should lead toward the intake side. After installation, inspect the hose routing and confirm that the valve's marked direction matches the actual flow path.

3. Mistake: Using restrictive fittings

   An undersized barb, sharp adapter, kinked hose, or overly restrictive valve can create excessive pressure drop. That resistance hinders PCV flow and can prevent the crankcase from ventilating as intended. A catch can only helps when vapor can move through the system without unnecessary restriction. Fix: Match fittings to the hose and intended PCV flow, keep bends smooth, and avoid adapters that reduce the passage abruptly. If the system develops unusual crankcase pressure after installation, inspect every fitting before assuming the engine has a mechanical fault. Crawford's guide to maintaining your Subaru PCV system provides useful context for checking the full ventilation path.

4. Mistake: Treating every WRX routing diagram as interchangeable

   EJ and FA engines do not use identical factory ventilation layouts. Fix: Start with the correct engine-specific routing diagram, then place the check valve according to that system's pressure paths rather than copying a generic hose diagram. Check valve placement is critical because it keeps crankcase vapor directed correctly under both boost and vacuum conditions.

Catch Can Routing vs a Crawford AOS: Which Controls Oil Mist Better?

Both systems address the same problem: crankcase vapors can carry oil mist back toward the intake. The difference is what happens after that vapor enters the separator. A catch can is a collection point. A Crawford Air Oil Separator is a continuous separation and return system designed for drivers who want consistent control without adding a drain interval to their maintenance routine.

A properly routed catch can can be an effective budget or DIY solution, but it depends on correct line selection, unrestricted flow, and regular attention. On a WRX, the right wrx oil catch can routing also depends on whether you are working with an EJ or FA engine and how the factory PCV system behaves under vacuum and boost. A can that is installed in the wrong branch, or that becomes full, can undermine the ventilation system rather than improve it.

These distinctions are grounded in the separation principle described in this government technical reference. Crawford Performance pioneered the automotive AOS category. Applying that principle to Subaru crankcase ventilation with a system that returns separated oil rather than asking the owner to dispose of it later. That makes an AOS especially compelling when the car sees sustained boost, track sessions, or long service intervals.

For a practical look at the hardware and routing decisions, review the guide to installing a WRX engine bay AOS. If your priority is a lower-cost DIY setup, choose a can with adequate baffling, route it through the correct PCV or breather circuit, and schedule inspections. If your priority is durable, low-maintenance oil mist control, compare the available Crawford Air Oil Separators to your engine and use case.

Contact Crawford Performance today to choose the right ventilation setup for your build.

Frequently Asked Questions

How should a WRX oil catch can be routed?

Route the can in the specific crankcase ventilation path you intend to separate. With the crankcase side connected to the can inlet and the intake side connected to the outlet. EJ engines commonly use the PCV line and the joined valve-cover breather path, while FA engines require closer attention to their different factory flow pattern. Avoid restrictive fittings that add unnecessary pressure drop to the PCV system. For broader background, see Crawford's Subaru crankcase ventilation system guide.

Does a WRX need an oil catch can?

Not every WRX needs one, but a catch can can reduce the amount of oil mist and crankcase vapor returning to the intake. It is especially useful when you want a serviceable, visible way to inspect what the ventilation system is carrying. The correct setup depends on the engine, modifications, driving use, and whether the can is maintained regularly.

Which side is in and out on an oil catch can?

The inlet is the side receiving vapor from the crankcase or PCV source. The outlet returns cleaned vapor toward the intake. Follow the markings on the specific can, and confirm the hose path before starting the engine. Reversing the flow can reduce separation performance and may interfere with crankcase pressure control.

Can you run a catch can without a check valve?

Do not remove a check valve when the routing requires one. A correctly oriented one-way valve helps prevent backflow during changes between vacuum and boost. A missing or reversed valve can allow boost to pressurize the crankcase or pull oil through the system. Crawford identifies check valves as critical to keeping vapor moving in the intended direction, not as optional plumbing.

How often does a WRX catch can need to be drained?

Drain intervals depend on the engine and use, so inspect the can regularly rather than assuming a fixed schedule. Catch cans require manual drainage of the oil and contaminants they collect, while a properly designed AOS returns separated oil to the crankcase automatically. See Crawford's Subaru oil catch can guide when choosing between the two systems.

Ready to Plan the Right Subaru Ventilation Setup?

The correct routing depends on your engine, boost strategy, and whether you want a manually drained catch can or a drain-back AOS. A quick fitment conversation can help you avoid reversed lines, poor drain placement, and check valve problems. Contact Crawford Performance to spec an oil catch can, drain-back AOS, or complete plumbed kit for your WRX.