Vacuum System Test

WARNING: This page is about a different car, the 1996 Buick Regal. However, it is still accessible from the selected car via links, so may be relevant.

NOTE: Before performing this test, perform OUTLET AIRFLOW INCORRECT test.

Start engine. Check vacuum supply to vacuum/electric solenoid box (Black vacuum hose). Vacuum should be about 18 in. Hg and remain at 18 in. Hg when engine is turned off.
Check each vacuum actuator by disconnecting vacuum hose and applying vacuum directly to actuator. Repair or replace actuator as necessary. See Fig 1.
Reconnect all vacuum and electrical connections. Run engine and cycle through all A/C-heating operating modes while observing vacuum actuators. Check for vacuum to actuators by observing vacuum actuator position. When vacuum is applied, actuator will retract.
If an actuator retracts when it should not, replace A/C-heater control panel. If an actuator does not retract, check for vacuum leaks, collapsed vacuum hoses or binding valves. If components are okay, replace vacuum/electric solenoid box.

Fig 1: Air Delivery Vacuum Distribution Chart

Courtesy of GENERAL MOTORS CORP.