9000 L4-1985cc 2.0L DOHC Turbo EFI (1987)
Charge Air Bypass Valve: Description and Operation
The function of the air bypass valve is to redirect turbo pressure in the inlet system when the throttle is closed quickly. The valve is operated using
engine vacuum against a spring loaded diaphragm. When the throttle is closed suddenly while the turbo is generating boost pressure, the valve opens to
divert the pressure back to the suction side of the turbo. This is done to prevent pressure waves from bouncing off the closed throttle plate causing
surging or noise when the throttle is reopened.