Air/Fuel Control

Three types of fuel control systems are used on General Motors vehicles. Descriptions of each system is as follows:

• Mixture Control Solenoid - Solenoid is spring loaded in the normally open (full fuel) delivery mode. When ignition is turned on, current is supplied to solenoid. The ECM controls air/fuel ratio by supplying a ground for the M/C solenoid. This energizing process occurs 10 times per second. The air/fuel ratio is varied by controlling the length of time solenoid is energized during each of the 10 on-off cycles. More "on" time (high dwell) will produce a leaner mixture, while less "on" time (low dwell) will deliver a richer mixture. In "open loop" mode, dwell will be fixed. Solenoid "on" time may be measured in degrees using a dwell meter set on the 6-cylinder scale.
• Throttle Body Injector - Injector is an electrical solenoid located in throttle body (dual injectors are used on 4.3L, 5.0L and 5.7L engines) which receives current when ignition is on. The ECM controls air/fuel mixture by regulating "on" time (pulse width) of injector. The ECM accomplishes this by pulsing the ground circuit for injector. ECM uses tach (RPM) signal to determine when to pulse injector (one time for each distributor reference pulse, alternately on dual injector systems) except during cranking, "clear flood" mode, deceleration and heavy acceleration, when fuel delivery is controlled by internal ECM calibration.
• Port Fuel Injectors - Injectors are electrical solenoids (located in intake manifold fuel rails) which supply fuel to individual intake valves. Injectors receive current from ignition switch and are energized by ECM when it provides a ground. There are 2 types of port fuel injection systems: simultaneous double-fire injection and sequential injection. Simultaneously double-fire injection sprays injectors once for each crankshaft rotation. Injectors deliver 1/2 the required fuel during each injector pulse. This provides the required fuel for each combustion cycle (every 2 crankshaft rotations). Sequential fuel injection fires injectors in engine spark plug firing order, using individual ground circuits which are controlled by ECM.