Fuel Delivery and Air Induction: Description and Operation

The ECM determines proper air/fuel mixture for varying engine operating conditions by monitoring vehicle sensor signals. The ECM then adjusts air/fuel mixture by varying the amount of time the Mixture Control (MC) solenoid is energized (solenoid dwell)

CLOSED LOOP OPERATION
During normal idle and part throttle operation this system operates in Closed Loop mode.
- In closed loop the ECM varies the MC solenoid dwell in response to the oxygen sensor input.

OPEN LOOP OPERATION
Under certain operating conditions--"Open Loop" modes--the ECM sends a fixed dwell signal to the MC solenoid.

- During cranking--below a certain rpm--the system is in "inhibit" mode and the dwell signal remains fixed for up to 25 seconds, then drops to zero until the 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 the engine is operated at wide open throttle the ECM provides a fixed enrichment signal to the MC solenoid.

CHECKING MC SOLENOID DWELL
During Closed Loop operation dwell should fluctuate between 10° and 50°.
- The amount of dwell fluctuation is not important, but the reading must fluctuate constantly.

- 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.

NOTE: Most dwell meters should be compatible with this system. If engine operation changes when a dwell meter is connected to the mixture control solenoid, use a different meter. Meters with three leads are more likely to be able to measure solenoid dwell correctly than meters with two leads.