Air/Fuel Ratio Control System
The air/fuel ratio control system is designed to control the air/fuel ratio in 2 ways. These are open loop and closed loop control, which are switched back and forth depending on signals from the following sensors; coolant temperature, engine RPM, battery voltage, exhaust gas and manifold vacuum.
Open-loop control takes place during engine starting, when engine is cold, during hot-restart, during deceleration, at high speeds and at full throttle. During open-loop air/fuel ratio is determined by the ECU to maximize driveability performance.
During closed-loop control (air/fuel ratio feedback control) the ECU varies air/fuel ratio depending upon feedback from the exhaust gas sensor (and barometric pressure sensor California models) and other basic sensor signals. The ECU adjusts air/fuel ratio mixture to stay within the parameters of the 3-way catalytic converter. However, during engine warm up this exhaust sensor signal is disregarded.
The air/fuel mixture is controlled by an air/fuel solenoid within the carburetor. The air-fuel solenoid valve opens or closes the compensating air bleed and main jet to compensate for richness or leanness of the air-fuel ratio. See Fig 1 . The richer solenoid valve is controlled by the ECU. During closed loop, fuel ratio mixture is controlled by the vacuum piston and richer solenoid valve. Idle speed is controlled by the idle speed actuator until closed loop mode operation is obtained.