Fuel Metering Modes of Operation

The Powertrain Control Module (PCM) monitors voltages from several sensors in order to determine how much fuel to give the engine. The PCM controls the amount of fuel delivered to the engine by changing the fuel injector pulse width. The fuel is delivered under one of several modes.

Starting Mode
When the ignition is first turned ON, the PCM energizes the fuel pump relay for 2 seconds. This allows the fuel pump to build pressure in the fuel system. The PCM calculates the air/fuel ratio based on inputs from the Engine Coolant Temperature (ECT), Mass Air Flow (MAF), Manifold Absolute Pressure (MAP), and Throttle Position (TP) sensors. The system stays in starting mode until the engine speed reaches a predetermined RPM.

Clear Flood Mode
If the engine floods, clear the engine by pressing the accelerator pedal down to the floor and then crank the engine. When the TP sensor is at Wide Open Throttle (WOT), the PCM reduces the fuel injector pulse width in order to increase the air to fuel ratio. The PCM holds this injector rate as long as the throttle stays wide open and the engine speed is below a predetermined RPM. If the throttle is not held wide open, the PCM returns to the starting mode

Run Mode
The run mode has 2 conditions called Open Loop and Closed Loop. When the engine is first started and the engine speed is above a predetermined RPM, the system begins Open Loop operation. The PCM ignores the signal from the Heated Oxygen Sensor (HO2S). The PCM calculates the air/fuel ratio based on inputs from the ECT, MAF, MAP, and TP sensors. The system stays in Open Loop until meeting the following conditions:
^ The HO2S has varying voltage output, showing that it is hot enough to operate properly.
^ The ECT sensor is above a specified temperature.
^ A specific amount of time has elapsed after starting the engine.

Specific values for the above conditions exist for each different engine, and are stored in the Electrically Erasable Programmable Read-only Memory (EEPROM). The system begins Closed Loop operation after reaching these values. In Closed Loop, the PCM calculates the air/fuel ratio (injector ON time) based upon the signal from various sensors, but mainly from the HO2S. This allows the air/fuel ratio to stay very close to 14.7:1.

Acceleration Mode
When the driver pushes on the accelerator pedal, air flow into the cylinders increases rapidly. To prevent possible hesitation, the PCM increases the pulse width to the injectors to provide extra fuel during acceleration. This is also known as power enrichment. The PCM determines the amount of fuel required based upon the TP, the coolant temperature, the MAP, the MAF, and the engine speed.

Deceleration Mode
When the driver releases the accelerator pedal, air flow into the engine is reduced. The PCM monitors the corresponding changes in TP, MAP, and MAF. The PCM shuts OFF fuel completely if the deceleration is very rapid, or for long periods, such as long, closed-throttle coast-down. The fuel shuts OFF in order to prevent damage to the catalytic converters.

Battery Voltage Correction Mode
When the battery voltage is low, the PCM compensates for the weak spark delivered by the ignition system in the following ways:
^ Increasing the amount of fuel delivered
^ Increasing the idle RPM
^ Increasing the ignition dwell time

Fuel Cutoff Mode
The PCM cuts OF- fuel from the fuel injectors when the following conditions are met in order to protect the powertrain from damage and improve driveability:
^ The ignition is OFF. This prevents engine run-on.
^ The ignition is ON but there is no ignition reference signal. This prevents flooding or backfiring.
^ The engine speed is too high, above red line.
^ The vehicle speed is too high, above rated tire speed.
^ During an extended, high speed, closed throttle coast down. This reduces emissions and increases engine braking.
^ During extended deceleration, in order to prevent damage to the catalytic converters

Fuel Trim
The powertrain control module (PCM) controls the air/fuel metering system in order to provide the best possible combination of driveability, fuel economy, and emission control. The PCM monitors the heated oxygen sensor (HO2S) signal voltage while in Closed Loop and regulates the fuel delivery by adjusting the pulse width of the injectors based on this signal. The ideal fuel trim values are around 0 percent for both short and long term fuel trim. A positive fuel trim value indicates the PCM is adding fuel in order to compensate for a lean condition by increasing the pulse width. A negative fuel trim value indicates that the PCM is reducing the amount of fuel in order to compensate for a rich condition by decreasing the pulse width. A change made to the fuel delivery changes the long and short term fuel trim values. The short term fuel trim values change rapidly in response to the HO2S signal voltage. These changes fine tune the engine fueling. The long term fuel trim makes course adjustments to fueling in order to re-center and restore control to short term fuel trim. A scan tool can be used to monitor the short and long term fuel trim values. The long term fuel trim diagnostic is based on an average of several of the long term speed load learn cells. The PCM selects the cells based on the engine speed and engine load. If the PCM detects an excessively lean or rich condition, it will set a fuel trim Diagnostic Trouble Code (DTC).