The 2019 Subaru WRX STI uses the legendary EJ257 2.5L turbocharged Boxer engine producing 310 horsepower. When P0299 (Turbo/Supercharger Underboost) appears, the engine isn't making requested boost pressure. Boost leaks are a common cause that can be found and fixed with careful inspection.
Understanding the EJ257 Boost System
The STI's turbo system routes pressurized air from the turbocharger through the intercooler (top-mount on the STI) and into the intake manifold. Numerous connections, couplers, and hoses can develop leaks that allow boost pressure to escape before reaching the cylinders.
Common Boost Leak Locations
Intercooler connections are frequent leak points. The silicone couplers connecting the intercooler to the inlet and outlet pipes can loosen, crack, or come off entirely. The top-mount intercooler's Y-pipe connections are particularly prone to issues.
Bypass valve (BPV) leaks allow boost to vent inappropriately. The BPV recirculates air during throttle lift, but if it leaks or sticks open, boost pressure escapes. Aftermarket BOVs sometimes don't seal properly.
Intake manifold leaks at the gaskets or throttle body connection cause boost loss. These leaks may also cause rough idle and lean conditions.
Turbocharger inlet pipe connections can develop cracks or loose clamps. The path from air filter to turbo and turbo to intercooler has multiple potential leak points.
Finding Boost Leaks
Pressurize the intake system using a boost leak tester or DIY method (blocking intake and applying shop air through a vacuum port). Listen for hissing or spray soapy water to find bubbles indicating leaks.
Visual inspection sometimes reveals obvious issues—disconnected hoses, cracked couplers, or loose clamps. Check all connections and clamps in the boost path.
Fixing Common Leaks
Tighten or replace loose clamps—T-bolt clamps provide better security than worm-gear clamps for boost applications. Replace cracked or worn silicone couplers. Ensure BPV/BOV is functioning and sealing properly. Replace intake manifold gasket if that connection is suspect.