Modes of Operation
MODES OF OPERATIONThe control module monitors voltages from several sensors to determine how much fuel to give the engine. The fuel is delivered under one of several conditions called "modes." All the modes are controlled by the control module.
STARTING MODE
When the key is first turned "ON," the control module turns "ON" the fuel pump relay for two seconds, and the fuel pump builds up pressure to the Central MFI unit. The control module checks the Engine Coolant Temperature (ECT) sensor, Intake Air Temperature (IAT) sensor, Throttle Position (TP) sensor, Manifold Absolute Pressure (MAP) sensor, and ignition signal, then determines the proper air/fuel ratio for starting. This ranges from 1.5:1 at -36° C (-33° F) to 14.7:1, at 94° C (201° F) running temperature.
The control module controls the amount of fuel delivered in the starting mode by changing how long the injector is turned "ON" and "OFF." This is done by "pulsing" the injector for very short times.
CLEAR FLOOD MODE
If the engine floods, clear it by pushing the accelerator pedal down all the way. The control module then pulses the injector at a 16.5:1 air/fuel ratio, and holds this injector rate as long as the throttle stays wide open, and the engine is below 600 RPM. If the throttle position becomes less than 65%, the control module returns to the starting mode.
RUN MODE
The run mode is the mode under which the engine operates most of the time. In this mode, the engine operates in one of two conditions:
- "Open Loop".
- "Closed Loop".
"Open Loop"
When the engine is first started. and it is above 400 RPM, the system goes into "Open Loop" operation. In "Open Loop," the control module ignores the signal from the Heated Oxygen Sensor (HO2S), and calculates the air/fuel ratio based on inputs from the ECT sensor and the MAP sensor. The system stays in "Open Loop" until the following conditions are met:
- The H02S has varying voltage output, showing that it is hot enough to operate properly. (This depends on engine temperature.)
- The ECT sensor is above a specified temperature.
- A specific amount of time has elapsed after starting the engine.
A normal functioning system may go into "Open Loop" at idle if Heated Oxygen Sensor (HO2S) temperature drops below the minimum requirement to produce voltage fluctuation.
"Closed Loop"
The specific values for the above conditions vary with different engines. and are stored in the Programmable Read Only Memory (PROM). When these conditions are met, the systems goes into "Closed Loop" operation. In "Closed Loop," the control module calculates the air/fuel ratio (injector on-time) based on the signal from the H02S. This allows the air/fuel ratio to stay very close to 14.7:1.
ACCELERATION MODE
When the control module senses rapid changes in throttle position and manifold pressure, the system enters the acceleration mode and provides the extra fuel needed for smooth acceleration.
DECELERATION MODE
When deceleration occurs, the fuel remaining in the intake manifold can cause excessive emissions and backfiring. When the control module observes a fast reduction in throttle opening and a sharp decrease in manifold pressure, it causes the system to enter the deceleration mode by reducing the amount of fuel delivered to the engine. When deceleration is very fast. the control module cuts off fuel completely for short periods.
HIGHWAY FUEL MODE (SEMI-"CLOSED LOOP")
This mode comes into operation at highway speeds and its purpose is to improve fuel economy. For the control module to operate in this mode, it first must sense the correct engine coolant temperature, ignition control, canister purge activity and constant engine speed. During semi-"Closed-Loop" operation, there will be very little long term fuel trim (formerly known as "Block Learn") and short term fuel trim (formerly known as "Integrator"), and the oxygen sensor values will read below 100 millivolts.
DECEL EN-LEANMENT
On deceleration, the control module senses a high MAP vacuum (low voltage or kPa) and leans the fuel spray for emission reasons, but it should be noted that the control module can trigger this condition (Decel En-Leanment) while the vehicle is not moving.
DECEL EN-LEANMENT OPERATION
This mode of operation can be mis-diagnosed as a lean condition. The control module will run the system lean on decel, or if the MAP sensor senses a low voltage (high engine vacuum), with the vehicle standing still, it will lean out the fuel delivery. If it is noted while testing an control module system (with Tech I scan tool), and the transmission is in park, that the HO2S reading is low (usually below 100 mV) and the long term and short term fuel trim are both around the 128 counts, lower the engine speed to 1000 RPM. If the oxygen sensor and long term fuel trim numbers respond normally at this RPM, it's possible that the system was fooled into the decel en-leanment mode of operation. If the oxygen sensor and long term fuel trim numbers do not respond at the lower RPM readings. there are other problems with the vehicle.
BATTERY VOLTAGE CORRECTION MODE
When battery voltage is low. the control module can compensate for a weak spark delivered to the distributor by:
- Increasing injector on time.
- Increasing the idle RPM.
- Increasing ignition dwell time.
FUEL CUTOFF MODE
No fuel is delivered by the injector when the ignition is "OFF" This prevents dieseling. Also, fuel is not delivered if no reference pulses are seen from the Distributor Ignition (DI) system, which means the engine is not running. Fuel cutoff also occurs at high engine RPM, to protect internal engine components from damage. Vehicle speed also enables fuel cutoff.