Modes of Operation

The control module monitors the voltages from several sensors in order to determine how much fuel to give the engine. The fuel is delivered under one of several conditions called modes. The control module controls all the modes.

STARTING MODE
When the key is first turned ON, the control module turns on the fuel pump relay for 2 seconds, and the fuel pump builds up pressure to the throttle body. The control module checks the Engine Coolant Temperature (ECT) sensor, the Intake Air Temperature (IAT) sensor, the Throttle Position (TP) sensor, the Manifold Absolute Pressure (MAP) sensor, and the ignition signal. Then the control module determines the proper air to fuel ratio for starting. This ranges from 1.5:1 at -36°C (-33°F) to 14.6:1, at 94°C (201°F) running temperature.

CLEAR FLOOD MODE
If the engine floods, clear the engine by depressing the accelerator pedal down to the floor. The control module then pulses the injector at a 16.5:1 air to fuel ratio. The control module 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 either Open Loop or Closed Loop.

OPEN LOOP
When the engine is first started and it is above 400 RPM, the system goes into the Open Loop operation. In the Open Loop, the control module ignores the signal from the HO2S, and the control module calculates the air to fuel ratio based on the inputs from the Engine Coolant Temperature (ECT) sensor and Manifold Absolute Pressure (MAP) sensor.

The system stays in Open Loop until the following conditions are met:

1. The HO2S has varying voltage output, showing that it is hot enough to operate properly. This depends on engine temperature.
2. The Engine Coolant Temperature (ECT) sensor is above a specified temperature.
3. A specific amount of time has elapsed after starting the engine.

A normal functioning system may go into an Open Loop at idle if the Heated Oxygen Sensor (HO2S) temperature drops below the minimum requirement to produce the voltage fluctuation.

CLOSED LOOP
The specific values for the above conditions vary with different engines. The Electronically Erasable Programmable Read Only Memory (EEPROM) stores the values. When these conditions are met, the systems goes into a Closed Loop operation. In a Closed Loop, the control module calculates the air to fuel ratio (injector on-time) based on the signal from the HO2S. This allows the air to fuel ratio to stay very close to 14.6:1.

ACCELERATION MODE
When the control module senses rapid changes in the throttle position and the manifold pressure, the system enters the acceleration mode. The system 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 the throttle opening and a sharp decrease in the manifold pressure, the control module 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 the fuel completely for short lengths of time.

HIGHWAY FUEL MODE (SEMI-CLOSED LOOP)
This mode comes into operation at highway speeds. The mode's purpose is to improve the fuel economy. For the control module to operate in this mode, the control module first must sense the correct engine coolant temperature, ignition control, canister purge activity, and constant engine speed. During the semi-Closed-Loop operation, there will exist the following items:

- Very little long term fuel trim (formerly known as Block Learn.)
- Short term fuel trim (formerly known as Integrator.)
- The heated oxygen sensor values reading below 100 millivolts.

DECEL EN-LEANMENT
On deceleration, the control module senses a high MAP vacuum (low voltage or kPa). Because of this the control module leans the fuel for emission reasons. Note that the control module can trigger this condition (decel en-leanment) while the vehicle is not moving.

DECEL EN-LEANMENT OPERATION
The VCM can misdiagnose the Decel-En-leanment mode of operation as a lean condition. The control module runs the system lean on decel, or if the MAP sensor senses a low voltage (high engine vacuum), with the vehicle standing still, the control module leans out the fuel delivery.

If the technician notes, while testing a control module system (with a scan tool) with the transmission in park, that the HO2S reading is low (usually below 100 mV), the long term and short term fuel trims are both around 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 heated oxygen sensor and long term fuel trim numbers do not respond at the lower RPM readings, other problems exist 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 the following items:

- The injector ON time.
- The idle RPM.
- The ignition dwell time.

FUEL CUTOFF MODE
No fuel is delivered by the injector when the ignition is off. This prevents dieseling. Also, the fuel is not delivered if no reference pulses are seen from the Ignition system, which means the engine is not running. The fuel cutoff also occurs at high engine RPM. In order to protect the internal engine components from damage,the control module disables the fuel when the vehicle speed reaches a specified speed.

NOTE: The engine is at the Operating Temperature 92°C to 104°C (196°F to 222°F).

CONVERTER PROTECTION MODE
The Control Module constantly monitors engine operation and estimates conditions that could result in high converter temperatures. If the Control Module determines the converter may overheat, it causes the system to return to Open Loop operation and enriches the fuel mixture.