Variable Induction System: Description and Operation

Fig. 45 Air Intake Tandem Control System:

The tandem valve is employed to improve atomization of fuel which is injected by the main fuel injector in response to various engine operating conditions.
When the tandem valve control solenoid valve, mounted underneath the air intake chamber, is de-activated, venturi vacuum is not applied to the diaphragm chamber of the tandem valve control diaphragm, so the tandem valve is nearly closed. The narrow clearance between the tandem valve and the inner wall of the throttle body generates a rapid air flow which promotes atomization of the injected fuel from the main fuel injector.
When the tandem valve control solenoid valve is activated, venturi vacuum is applied on the tandem valve control diaphragm and the tandem valve is opened in response to venturi vacuum which represents the air flow rate through the venturi. Therefore good atomization of the injected fuel is provided regardless of the air flow rate.