9000 L4-1985cc 2.0L DOHC (1986)
Air Bypass Valve: All Technical Service Bulletins
Turbocharger Air Bypass Valve - Problem Diagnosis
CATEGORY Engine
SECTION
PAGE
2
5
ISSUE
CODE
10/90-0034
291
Subject:
Diagnosing a Leaking Air
Bypass Valve
Application:
900 and 9000 Turbo Models, 1985-
If a 900 or 9000 Turbo model develops a complaint of driveability problems or even engine stalling, it may be the result of a leaking diaphragm within
the air bypass valve. In order to help diagnose a problem related to the air bypass valve, it is essential to know the function of the valve and how it
operates.
Bypass Valve Function
The function of the turbo 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 enclosed 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.
Bypass Valve Operation-Throttle Open
Whenever the throttle is open and the turbo is generating boost, positive pressure is present inside the intercooler, delivery pipes, throttle housing, and
the intake manifold. At the same time, a vacuum is formed in the inlet pipe between the air cleaner and the compressor side of the turbo.
Figure 1
As long as the throttle is open, boost pressure acts on both sides of the diaphragm in the valve. The valve is held closed by the combined effort of the