Fuel Control System

The Single Overhead Cam (SOHC) and Dual Overhead Cam (DOHC) engines utilize Sequential Fuel Injection (SFI). SFI allows the PCM to individually control each fuel injector which optimizes fuel economy and performance. The PCM Pulse Width Modulates (PWM) each fuel injector by individually pulling each fuel injector circuit to ground.

The PCM bases its fuel injector pulse width (the amount of fuel the engine needs) on three main parameters:
^ Temperature of the air/fuel mixture at the intake valve. Calculation is based on the Engine Coolant Temperature (ECT) sensor and Intake Air Temperature (IAT) sensor.
^ Engine speed from the Crankshaft Position (CKP) sensor
^ Engine load. Calculation is based on the Manifold Absolute Pressure ([1][2]MAP) sensor and the Throttle Position (TP) sensor. The TP sensor is used to ensure a correct load value if throttle change is rapid, however, the MAP sensor is primarily used.

These parameters allow the PCM to calculate a base fuel injector pulse width when the system is in open loop. Open loop is when the PCM is not using the Oxygen Sensor 1 (O2S-1) to modify fuel. The system is in open loop anytime the engine is running and the O2S-1 is not varying above 600 mV or below 300 mV The O2S-1 signal on Scan tool will read between 391 and 491 mV when cold.

The PCM will go into open loop fuel control if a fault exists in the O2S-1 circuit. If a fault exists in one of the sensors necessary to calculate the base pulse width, the PCM will attempt to calculate a modeled value for substitution into the calculation. However, the Scan tool will still read the real sensor signal.

When the O2S-1 is above 600°F, it produces a voltage based on the amount of oxygen in the exhaust gas. The PCM can make fuel corrections to achieve a desired 14.7 to 1 air/fuel ratio by using the O2S-1. Normal readings vary between 10 and 999 mV When the PCM detects the O2S-1 voltage above 600 mV or under 300 mV for a short period of time, it will go into closed loop fuel control. When the exhaust gas has high oxygen content, the air/fuel mixture is lean and the O2S-1 signal voltage will be low. To compensate, the PCM will command rich or increase the fuel injector pulse width. When the exhaust gas has low oxygen content, the air/fuel mixture is rich and the O2S-1 signal voltage will be high. To compensate, the PCM will decrease the amount of fuel by reducing the injector pulse width.

The PCM has the ability to adapt fuel control based on previous O2S-1 signals. The Short Term Fuel Trim (STFT) value is used to adapt fuel control over a short period of time. A value of 128 is the nominal STFT value the engine should be running at. If the engine is running at 128 in closed loop, the PCM does not have to modify fuel to obtain a 14.7 to 1 air/fuel ratio. The 128 value is based off of the calculation from the three main parameters. If for instance the vehicle is running rich, the STFT value will decrease causing the PCM to decrease the injector pulse width. The PCM will continue to do this until the O2S-1 indicates a lean condition. The PCM will toggle from rich to lean to improve catalytic converter efficiency.

The Long Term Fuel Trim (LTFT) values are an average of the STFT values. There are four different LTFT cells: decel, idle, cruise and accel. When the vehicle is in one of these conditions, it will use the LTFT adaptive fuel correction value that it has stored. For instance, the vehicle could be running lean at idle, but be rich while cruising. So if the vehicle is cruising then comes down to idle, the PCM will automatically increase the injector pulse width according to the idle LTFT cell value. The PCM also uses a different set of cells when the EVAP purge solenoid is commanded On to purge fuel vapors from the EVAP canister to the intake manifold.

To obtain a reading of how the vehicle is running overall, the LTFT values should be used. To obtain a reading of how the vehicle is running at a particular instant, the STFT value should be used. The STFT and LTFT values can significantly aid in diagnosing a drive ability concern if used properly.