Ford Mustang/Capri 1979-1988 Repair Guide

Venturi Vacuum Amplifier System



See Figures 1 and 2

Many models use a venturi vacuum amplifier in conjunction with the EGR system. The amplifier is used to boost a relatively weak venturi vacuum signal (in the throat of the carburetor or throttle body) into a strong intake manifold vacuum signal in order to operate the EGR valve. This device improves driveability by more closely matching venturi airflow and EGR flow.

The amplifier features a vacuum reservoir and check valve to maintain an adequate vacuum supply regardless of variations in engine manifold vacuum. Also used in conjunction with the amplifier is a relief valve, which will cancel the output EGR vacuum signal whenever the venturi vacuum signal is equal to, or greater than, the intake manifold vacuum. Thus, the EGR valve may close at or near wide-open throttle acceleration, when maximum power is needed.

Click image to see an enlarged view

Fig. Fig. 1: EGR venturi vacuum amplifier

Click image to see an enlarged view

Fig. Fig. 2: A common EGR system with venturi vacuum amplifier


  1. Connect a vacuum gauge to port O of the amplifier. Do not remove the connection at port R. The gauge may read as much as 2 in. Hg (6.75 kpa) at idle.
  3. Disconnect the venturi vacuum hose at the carburetor or throttle body, and increase engine speed to 2000 rpm (3000 for the 4-140 engine). Vacuum should not change.
  5. Maintain a high engine speed and connect the venturi vacuum hose. The gauge should read at least 4 in. Hg (13.5 kpa).
  7. Return to idle. The gauge should return to its initial reading.
  9. If the above conditions are not met, replace the venturi vacuum amplifier.


  1. Tag and disconnect the vacuum hoses at the venturi vacuum amplifier.
  3. Remove the fasteners which secure the venturi vacuum amplifier and remove the unit.

To install:
  1. Installation is the reverse of removal. Be sure to connect the vacuum hoses to their proper ports. Replace any vacuum hoses which are cracked or brittle.