Fuel Control System Operation: Notes

The function of fuel metering system is to deliver correct amount of fuel to engine under all operating conditions. Fuel is delivered to engine by individual fuel injectors mounted into intake manifold near each cylinder. Main fuel control sensors are Manifold Absolute Pressure (MAP) sensor, Oxygen (O2S) sensor, and Heated Oxygen sensor (HO2S).

The MAP sensor measures or senses intake manifold vacuum. Under high fuel demands, MAP sensor reads a low vacuum condition, such as wide open throttle. The Engine Control Module (ECM) uses this information to enrich mixture, thus increasing fuel injector on-time, to provide correct amount of fuel. When decelerating, vacuum increases. This vacuum change is sensed by MAP sensor and read by ECM, which then decreases fuel injector on-time due to low fuel demand conditions.

The O2S sensor is located in exhaust manifold. HO2S is located in exhaust pipe. Oxygen sensors indicate to ECM amount of oxygen in exhaust gas, and ECM changes air/fuel ratio to engine by controlling fuel injectors. The best air/fuel ratio to minimize exhaust emissions is 14.7:1, which allows catalytic converter to operate most efficiently. Because of constant measuring and adjusting of air/fuel ratio, fuel injection system is called a "closed loop" system.

The ECM uses voltage inputs from several sensors to determine how much fuel to provide to engine. The fuel is delivered under one of several conditions, called modes.