Modes of OperationThrottle Actuator Control (TAC) System Description

The engine is fueled by individual fuel injectors, one for each cylinder, that are controlled by the engine control module (ECM). The ECM controls each fuel injector by turning ON the fuel injector coil for a brief period once every other engine revolution. The period of time when the fuel injector is turned ON is called the pulse width and is measured in milliseconds. The pulse width is calculated by the ECM to deliver the correct amount of fuel for proper driveability and emissions control.
While the engine is running, the ECM is constantly monitoring the inputs and recalculating the appropriate pulse width for each fuel injector. The pulse width calculation is based on the fuel injector flow rate, the desired air/fuel ratio, and actual air mass in each cylinder. The pulse width is also adjusted for battery voltage, short term, and long term fuel trim. The fuel injector pulse, is timed to occur as each cylinders intake valves are closing to attain largest duration and most vaporization.
The fueling system has several automatic adjustments in order to compensate for the differences in fuel system hardware, driving conditions, fuel used, and vehicle aging. The battery voltage adjustment is necessary since the changes in the voltage across the fuel injector affect the flow rate. The short term and the long term fuel trims are fine and gross adjustments to the pulse width that are designed to maximize driveability and emissions control. These fuel trims are based on the feedback from the oxygen sensors in the exhaust stream and are only used when the fuel control system is in Closed Loop operation.
Fueling during cranking is slightly different than fueling during an engine run. As the engine begins to turn, a prime pulse may be injected to speed starting. Once the ECM determines where the engine is in the firing order, the ECM begins pulsing the fuel injectors. The pulse width during cranking is based on the coolant temperature and the engine load.
Under certain conditions, the fueling system will turn OFF the fuel injectors for a period of time. This is referred to as fuel cut-off. Fuel cut-off is used in order to improve traction, save fuel, improve emissions, and protect the vehicle under certain extreme or abusive conditions.
In case of a major internal problem, the ECM may be able to use a back-up fuel strategy for limp in mode that will run the engine until service can be performed.

When the ECM detects reference pulses from the crankshaft position (CKP) sensor, the ECM will enable the fuel pump. The fuel pump runs and builds up pressure in the fuel system. The ECM then monitors the manifold absolute pressure (MAP), intake air temperature (IAT), engine coolant temperature (ECT), and the throttle position (TP) sensor signals in order to determine the required injector pulse width for starting.

If the engine is flooded with fuel during starting and will not start, the Clear Flood Mode can be manually enabled. To enable Clear Flood Mode, press the accelerator to wide open throttle (WOT). The ECM will completely turn OFF the fuel injectors and will maintain this mode as long as the ECM detects a WOT condition with engine speed below a predetermined value.

The Run Mode has 2 conditions called Open Loop operation and Closed Loop operation.

Upon initial engine start up and when the engine speed is more than a predetermined value, the ECM operates the fuel system in Open Loop operation. During Open Loop operation, the ECM ignores the signals from the oxygen sensors and calculates the required fuel injector pulse width based primarily on inputs from the MAP, IAT and ECT sensors. The ECM operates in Open Loop until the following conditions are met:

During Closed Loop operation, the fuel injector pulse width is based on the oxygen sensor signal. The ECM uses the oxygen sensor signal to keep the air/fuel ratio close to 14.7:1 as possible.

The ECM monitors the changes in the TP and the MAP sensor signals in order to determine when the vehicle is being accelerated. The ECM will then increase the injector pulse width in order to provide more fuel for improved performance.

The ECM monitors changes in TP and MAP sensor signals to determine when the vehicle is being decelerated. The ECM will then decrease injector pulse width or even turn OFF injectors for short periods to reduce exhaust emissions, and for better (engine braking) deceleration.

The ECM can compensate to maintain acceptable vehicle driveability when the ECM detects a low battery voltage condition. The ECM compensates by performing the following functions:

To prevent dieseling or engine run-on, no fuel is delivered to the fuel injectors when the ignition is OFF. To prevent flooding, no fuel is delivered to the fuel injectors when no reference pulses are received from the CKP sensor.