Modes of Operation

OPEN LOOP MODE (4-151 ENGINE)
The engine is in this mode of operation when:
a. Starting the engine and the engine is cold or air cleaner air is cold.
b. Engine is being decelerated to idle from partial throttle or is being accelerated to partial throttle from idle position.
c. Carburetor is at or near wide open throttle.

Each operation has a specific air/fuel ratio and because more than one of the engine operational selection conditions can be present at one time, the MCU is programmed with a priority ranking for the operations that comply with the operation having the highest priority. These priorities are described below.

Fig.11a Switch calibrations:

Fig.12a Switch calibrations:

Cold Weather Engine Start-up and Operation
If the air cleaner air temperature is below the calibrated value of the thermal electric switch (TES), Figs. 11a and 12a, the air/fuel mixture is at a rich ratio. Lean air/fuel mixtures are not used for a preset period following a cold weather start-up.

At Or Near Wide Open Throttle Operation (Engine Cold)
This mode of operation occurs whenever the coolant temperature is below the calibrated switching temperature of the open loop coolant temperature switch, Figs. 11a and 12a, and the wide open throttle vacuum switch (cold) has been closed due to a decrease in manifold vacuum. When the engine is in this mode of operation, the mixture control (MC) solenoid provides a rich air/fuel mixture for cold engine operation at wide open throttle.

At Or Near Wide Open Throttle Operation (Warm Engine)
This mode of operation occurs whenever the coolant temperature is above the calibrated switching temperature of the enrichment coolant temperature switch and wide open throttle vacuum switch (warm) has been opened because of the decrease in manifold vacuum. When the engine is in this mode of operation, the MC solenoid provides a rich air/fuel mixture for warm engine operation at wide open throttle.

Adaptive Mode Of Operation
This mode of operation occurs when the engine is either at idle speed, accelerating from idle speed or decelerating to idle speed. If the engine RPM (tach) voltage is less than the calibrated value and manifold vacuum is above the calibrated switching level for the adaptive vacuum switch (switch open), an engine-from acceleration-from idle speed condition is assumed to exist. If the engine rpm (tach) signal is greater than the calibrated value and manifold vacuum is above the calibrated switching level of the adaptive vacuum switch (switch closed), an engine-deceleration-to-idle speed condition is assumed to exist. During the adaptive mode of operation, the MC solenoid provides a predetermined air/fuel mixture.

CLOSED LOOP MODE

The engine is in this mode of operation when none of the open loop engine operating conditions exist. The MCU causes the MC solenoid to vary the air/fuel mixture in reaction to the voltage input from the oxygen content of the exhaust gases, Since the oxygen content of the exhaust gases indicates the completeness of the combustion process, it is a reliable indicator of the air/fuel mixture entering the combustion chamber.

During operation, the MCU causes the stepper motor to drive the metering pins to a leaner or richer position in reaction to the voltage input from the oxygen sensor. The stepper motor drives two moveable air metering pins in the carburetor to control the air/fuel ratio.

The engine operating characteristics almost never allow the MCU to compute a single metering pin position that constantly provides the optimum air/fuel mixture. Therefore, the closed loop operation is characterized by the constant movement of the metering pins because the MCU is constantly making corrections to the air/fuel mixture in an attempt to create an optimum air/fuel mixture ratio.