Diagnosis And Testing - Vacuum Supply Test

DIAGNOSIS AND TESTING - VACUUM SUPPLY TEST

4.7L Gas Powered Engines

4.7L gas powered engines only: actual engine vacuum, a vacuum reservoir, a one-way check valve and vacuum lines are used to supply vacuum to the speed control servo.

1. Disconnect vacuum hose at speed control servo and install a vacuum gauge into the disconnected hose.

2. Start engine and observe gauge at idle. Vacuum gauge should read at least ten inches of mercury.

3. If vacuum is less than ten inches of mercury, determine source of leak. Check vacuum line to engine for leaks. Also check actual engine intake manifold vacuum. If manifold vacuum does not meet this requirement, check for poor engine performance and repair as necessary.

4. If vacuum line to engine is not leaking, check for leak at vacuum reservoir. To locate and gain access to reservoir, refer to Vacuum Reservoir Removal/Installation. Disconnect vacuum line at reservoir and connect a hand-operated vacuum pump to reservoir fitting. Apply vacuum. Reservoir vacuum should not bleed off. If vacuum is being lost, replace reservoir.

5. Verify operation of one-way check valve and check it for leaks.
a. Locate one-way check valve. The valve is located in vacuum line between vacuum reservoir and engine vacuum source. Disconnect vacuum hoses (lines) at each end of valve.
b. Connect a hand-operated vacuum pump to reservoir end of check valve. Apply vacuum. Vacuum should not bleed off. If vacuum is being lost, replace one-way check valve.
c. Connect a hand-operated vacuum pump to vacuum source end of check valve. Apply vacuum. Vacuum should flow through valve. If vacuum is not flowing, replace one-way check valve. Seal the fitting at opposite end of valve with a finger and apply vacuum. If vacuum will not hold, diaphragm within check valve has ruptured. Replace valve.

3.7L V-6 / 5.7 V-8 Gas

Vacuum is not used for any part of the speed control system.

5.9L Diesel

Vacuum is not used for any part of the speed control system if equipped with a diesel engine.