Secondary Throttle Valve: Description and Operation

Purpose
This system prevents (cold engine) hesitation on acceleration by compensating for the manifold absolute pressure sensor's (MAP) response delay to abrupt changes in intake air flow (acceleration).

Operational Theory
The throttle body contains two throttle valves which are opened by a mechanical linkage. When both throttle valves are opened abruptly, the engine ingests a large amount of cold, dense air. The fuel injection system can not compensate for the dense air amount with a corresponding change in fuel supply; the result is a temporary lean air/fuel mixture and hesitation.

During cold engine operation, (coolant temp. below 80°C [176°F]), the powertrain control module engine (PCME) allows the Double Throttle Control System to control the actual secondary air flow through the throttle body via a vacuum operated throttle valve adjacent to the mechanically operated throttle valve. The delayed throttle valve opening provides the fuel injection system with enough time to compensate for the changes in air flow thus reducing cold engine hesitation on acceleration.