Fuel Control: Notes

The engine is equipped with a "feedback" carburetor which contains vacuum-operated fuel control actuators. The ECM, responding to inputs from the data sensors, constantly adjusts the air/fuel ratio to maintain engine performance. The ECM sends electrical signals to a vacuum control valve. The vacuum control valve converts the ECM signals to vacuum signals to operate the actuators.

The vacuum control valve consists of a vacuum regulator and a vacuum control solenoid. The regulator changes the inconsistent vacuum levels from the intake manifold into constant vacuum levels. The solenoid, controlled by the ECM, uses the vacuum from the regulator to operate the fuel control actuators.

When the ECM adjusts fuel mixture based upon signals received from the oxygen sensor, the system is in closed loop operation. Under certain operating conditions, the ECM may ignore inputs from various data sensors and use a pre-programmed calibration control to operate the engine. During cold engine starts, the vacuum control solenoid is turned off by the ECM to provide a rich mixture. Operating conditions which cause the ECM to ignore oxygen sensor signals cause the system to operate in the open loop mode.

Although not a major component of the fuel control system, a vacuum-operated mixture control (M/C) valve is mounted on the right side of the engine. The M/C valve prevents backfiring during deceleration by admitting air from air pump into manifold when the throttle valve is suddenly closed. The M/C valve is closed under normal operating conditions. When vacuum in the intake manifold increases rapidly, the valve opens, allowing air from the air pump into the intake manifold.