Fuel Delivery and Air Induction: Description and Operation

The ECM determines proper air/fuel mixtures for engine operating conditions by monitoring oxygen sensor and other vehicle sensor signals. Air/fuel mixture is controlled through the MC solenoid by varying the amount of time the solenoid is energized (dwell period) during each cycle. This is called ``Closed Loop'' operation. During some modes of operation, such as when engine is cold or operating at wide open throttle, the dwell period will remain as a fixed value. This is called ``Open Loop'' operation.
MC solenoid operation can be monitored using a dwell meter connected to the MC solenoid test lead and set on the 6 cylinder scale for all engines. Normal dwell reading during ``Closed Loop'' fluctuates between 10° and 50°. If leaner mixtures are required, reading will fluctuate toward the higher end of the scale. If richer mixtures are required, reading will fluctuate toward the lower end of the scale. For proper ``Closed Loop'' operation, the amount of dwell fluctuation is not important, but the reading must fluctuate constantly.
During normal idle and part throttle operation, this system operates in ``Closed Loop.'' However, under certain operating conditions the ECM sends a fixed dwell signal to the MC solenoid, and operates in ``Open Loop'' mode.
During cranking below a certain RPM, the system is in ``inhibit'' mode and the dwell signal will remain fixed for up to 25 seconds, then drop to zero until a specified RPM is reached. During engine warm-up or when oxygen sensor is cold, the dwell signal will remain fixed until the sensors reach specified operating temperature. When engine is operated at wide open throttle, the Throttle Position Sensor (TPS) sends a higher voltage signal to the ECM, which provides a fixed enrichment signal to the MC solenoid.