Vacuum Differential Valve (VDV)

With engine at normal operating temperature, transmission in park or neutral and parking brake set, disconnect bypass valve-to-exhaust check valve hose at bypass valve. Connect a tachometer to engine.
NOTE: For a tee system, disconnect hoses connecting both check valves to bypass valve.
Start engine, place a hand over bypass valve outlet connection and accelerate engine to 2500 RPM. Release throttle and allow engine to return to normal idle speed.
During the deceleration period, air flow at bypass valve outlet should be felt and/or heard to momentarily diminish or stop and then return to normal. Air flow will be discharged through exhaust ports in end of bypass valve silencer cover.
NOTE: The length of time required to resume normal air flow cannot be specified since the time interval is dependent on engine vacuum, and length of time vacuum line is pinched off.
If bypass valve is functioning properly, but does not operate as described in step 3, check to ensure that vacuum differential valve (VDV) is receiving vacuum. If vacuum is present, VDV is defective and should be replaced.