Service Repair Manual

GENERAL (FBC)
FBC System Operation
1. The air-fuel ratio control is maintained by the ECU in one
of two operating modes.
1) Closed loop control (Feedback control)
After engine warm-up, the air-fuel ratio control is made
by the feedback control based on the oxygen sensor
signal.
The oxygen sensor output voltage changes sharply at the
stoichiometric ratio. The control unit senses this oxygen
sensor signal and provides feedback control to the FBSV
maintaining the stoichiometric ratio that will give the
best purification rate of the 3-catalyst converter may be
accurately kept. In this state, the SCSV is kept wide open
(100% duty).
2) Open loop control (No feedback control)
During engine start, warm-up operation, high load
operation and deceleration, the air-fuel ratio is in
open-loop. The ECU controlled based on map values*
established previously for engine speed, throttle valve
opening angle and engine coolant temperature, to
improve startability and driveability.
During deceleration, the SCSV limits fuel flow for better
fuel economy and for prevention of overheating of the
catalysts.
* Map value is a value previously established and
stored in ROM in ECU.
2. When the FBSV is energized, the main fuel jet is closed to
leaving the primary main jet passage as the only fuel
passage.
This reduces the amount of fuel, resulting in leaner air-fuel
mixture.
3. When the FBSV is de-energized, the main fuel jet is opened
to provide two fuel passages including the primary main jet
passage. Since this will increase the amount of fuel, richer
air-fuel mixture is obtained.
With the ON-OFF operation of SCSV, the slow fuel passage
is opened and closed.
The air-fuel ratio at deceleration is controlled in this manner.
31-84