Secondary Throttle Valve: Description and Operation

Secondary Port Throttle Valves:

Electric Vacuum Pump:

DESCRIPTION
Each cylinder has two intake valves and two separate intake ports (primary and secondary), one for each valve. The primary intake ports flow air all the time the engine is running. The secondary ports allow air flow only when the engine is under heavy load (wide open throttle) and the driver has selected "FULL" power mode with the engine power mode switch on the instrument panel. The secondary port throttle valves are located in the secondary air intake ports in the cylinder head. They are normally closed to block off the secondary intake air-flow, and open to provide additional air flow to the cylinders during "FULL" power mode. An injector is installed in each secondary intake port to provide fuel for the additional air inducted during "FULL" power operation.

The valves are vacuum operated, and controlled by an ECM actuated solenoid, based on inputs from the engine power mode switch, and signals from the TPS, engine oil temperature sensor, Manifold Absolute Pressure (MAP) sensor, and ignition module (engine speed signal). Vacuum to operate the valve actuators is supplied by the engine and supplemented by a self-starting electric vacuum pump.

OPERATION
In operation, the secondary port throttle valves are closed during low engine speeds and loads, providing a single air path to each cylinder. This causes the air to enter the cylinders at a higher velocity, providing better fuel atomization for more complete combustion and improved low speed driveability. If the driver has the power switch in the FULL power position, and pushes down on the accelerator pedal far enough, and if various input sensor parameters are within an acceptable range, the ECM consecutively turns ON:

^ The secondary port throttle valve solenoid (allowing vacuum to open the valves).
^ The secondary injectors.

If system vacuum drops below 41 kPa (12" Hg) a switch within the electric vacuum pump will turn the pump ON to supplement engine vacuum. The ECM monitors vacuum to the valve actuators through a vacuum sensor teed in-line with the actuators and checks the vacuum sensor output signal against the commanded status of the system. During "FULL" power operation the ECM will set a Code 61 if vacuum to the actuators drops below 21 kPa (6" Hg). During "NORMAL" power operation (secondary operation disabled) a Code 61 will set if any vacuum is present at the valve actuators. The ECM also checks the vacuum sensor signal against stored values to detect open or shorted sensor circuit. If the sensor signal is out-of-range, a code 56 will set. If any trouble codes are stored, the ECM will disable the secondary port throttle/secondary injector operation.