Procedure
- Remove the intake between the airbox and turbo inlet, and fit the tester cap to the turbo inlet. Sealing at the inlet tests the entire pressurized tract at once.
- Regulate compressed air DOWN before connecting. Test at no more than your normal peak boost — most WRX/STI setups need nothing over 20 psi, and over-pressuring can pop seals that were fine.
- Pressurize slowly and listen. Big leaks announce themselves immediately; the usual suspects are the top-mount-to-throttle-body coupler, BPV hose, and the small vacuum lines under the intercooler.
- Spray soapy water on every coupler, clamp, and vacuum tee. Bubbles mark the leak. Work front to back so drips don't give false positives.
- Check the bypass/blow-off valve itself — a torn BPV diaphragm is a classic hidden leak on higher-mileage EJs and idles almost normally.
- Fix, re-clamp, and re-test until the system holds pressure with only the normal slow bleed-down past the turbo seals and open valves.
- Clear fuel trims (or let the ECU relearn) and confirm A/F corrections settle back near zero on a datalog.
Work at your own risk. Torque values and procedures vary by vehicle — always confirm against the factory service manual for your exact model.