Auxiliary Accelerator Pump System: Operation
When accelerating with a cold engine, the main acceleration pump capacity is insufficient to provide good acceleration. The AAP system is vacuum controlled and the VSV is open during constant RPM. Intake vacuum pulls on the AAP diaphragm drawing fuel into the AAP chamber. During acceleration, low vacuum applied to the AAP diaphragm allows spring tension to force fuel from the AAP chamber into the acceleration nozzle.
Courtesy of Toyota Motor Sales, U.S.A., Inc.