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.

Minimum Function System
Under most conditions this system will operate in ``Closed Loop.'' However, when oxygen sensor temperature is below 600° F (not transmitting signals to ECM) or coolant temperature is below sensor calibration point, the range of dwell control is restricted. System operation is determined by the amount of time engine has been running (oxygen sensor temperature) and coolant temperature. If there is uncertainty whether system is operating in ``Closed Loop,'' induce a large intake air leak while observing dwell readings. If reading goes below 10°, system is in ``Closed Loop'' mode.