Vacuum Differential Valve (VDV)

1. Start engine and allow to reach normal operating temperature with transmission in Park or Neutral and parking brake applied.
2. Stop engine and remove air bypass valve to air manifold check valve hose and bypass valve. On systems with two check valves, remove both hoses. Connect tachometer to engine.
3. Start engine, then with hand placed over bypass valve outlet connections, accelerate engine to 2500 RPM and release throttle, allowing engine to return to normal idle. During deceleration, the air flow in the valve outlet should be felt or heard to momentarily diminish or stop and then resume to normal air flow. Air flow will be discharged through the exhaust ports in the end of the bypass valve silencer cover.
4. If bypass valve does not function as described in step 3, check to make sure that decel valve is receiving a vacuum signal. If it is, the decel valve is defective and should be replaced.