Subaru Turbocharger Explained Part 2:

Air Bypass Valve Inspection

The air bypass valve is located downstream of the turbocharger and provides a bypass passage for the compressed air intake back to the inlet side of the turbocharger.When deceleration occurs immediately after a period of high engine load (high boost pressure), a large pressure differential occurs at the compressor wheel of the turbocharger.

This is due to the inertia of the turbocharger, which still generates boost pressure even though the throttle is fully closed. This high pressure may lead to increased noise, and possible damage to the turbocharger because of the high pressure that occurs in the compressor.

The upper chamber of the bypass valve is connected to the intake manifold and the negative pressure (vacuum) during deceleration opens the valve by acting on the diaphragm.

Operation of the bypass valve can be tested by attaching a hand-held vacuum pump to the intake manifold connection. Apply vacuum with the pump and confirm that the valve opens.