PCV valve failure in your 2018 Subaru Forester causes various driveability and emissions problems that may seem unrelated at first glance. The positive crankcase ventilation system maintains proper engine breathing, and when it fails, symptoms range from rough idle to increased oil consumption and check engine lights.
PCV System Function
The PCV valve regulates crankcase gases—blow-by from combustion past piston rings—routing them back into the intake for burning rather than releasing to atmosphere. The valve opens under vacuum to allow flow and closes under certain conditions to prevent backfire into the crankcase.
Stuck Open Symptoms
A PCV valve stuck open creates a constant vacuum leak at the crankcase. This causes lean running conditions, rough idle, and potential P0171 lean codes. The engine may run poorly at idle but improve at higher RPM where the leak becomes proportionally smaller.
Stuck Closed Symptoms
When the PCV valve sticks closed, crankcase pressure builds with no ventilation path. Oil is pushed past seals causing external leaks and consumption. The oil filler cap may pop off during running. Increased blowby accelerates oil degradation.
Boxer Engine Considerations
Subaru's boxer engine layout routes PCV system differently than inline engines. The horizontal cylinder orientation affects how crankcase gases and oil mist move through the system, making PCV maintenance particularly important for these engines.