The ECM 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 ECM controls all of the modes.
Starting Mode
When the key is first turned ON, the ECM turns ON the fuel pump relay for two seconds, and the fuel pump builds up pressure to the TBI unit. The ECM checks the Engine Coolant Temperature (ECT) sensor, the Throttle Position (TP) sensor, the Manifold Absolute Pressure (MAP) sensor, and the crank signal. The ECM 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 ECM 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 lengths of times.
Clear Flood Mode
If the engine floods, clear the engine by pushing the accelerator pedal down all the way. The ECM then pulses the injector at a 20:1 air/fuel ratio. The ECM 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 80 percent, the ECM returns to the starting mode.
Run Mode
The run mode has two conditions called Open Loop and Closed Loop.
Open Loop
When the engine first starts, and it is above 400 RPM, the system goes into an Open Loop operation. In Open Loop, the ECM ignores the signal from the oxygen sensor and calculates the air/fuel ratio based on inputs from the ECT and the MAP sensors. The system stays in Open Loop until the following conditions are met:
| • | The oxygen sensor has a varying voltage output, showing that the sensor is hot enough to operate properly. This depends on the temperature. |
| • | The ECT sensor indicates a coolant temperature above a specified temperature. |
| • | A specific amount of time has elapsed after starting the engine. |
Closed Loop
The specific values for the above conditions vary with different engines. The specific values are stored in the Programmable Read Only Memory (PROM). When these conditions are met, the systems goes into a Closed Loop operation. In Closed Loop, the ECM calculates the air/fuel ratio (injector on-time) based on the signal from the oxygen sensor. This allows the air/fuel ratio to stay very close to 14.7:1.
Acceleration Mode
The ECM looks at the rapid changes in the Throttle Position (TP) and the Manifold Absolute Pressure (MAP) and provides extra fuel.
Deceleration Mode
When deceleration occurs, the fuel remaining in the intake manifold can cause excessive emissions and backfiring. Again, the ECM looks at changes in the throttle position and the manifold pressure and reduces the amount of fuel. When deceleration is very fast, the ECM can cutoff the fuel completely for short periods.
Battery Voltage Correction Mode
When the battery voltage is low, the ECM can compensate for a weak spark delivered by the distributor by the following steps:
| • | Increasing the injector on time of fuel delivered |
| • | Increasing the idle RPM |
Fuel Cutoff Mode
The injectors do not defier any fuel when the ignition is OFF. This prevents dieseling. Also, fuel is not delivered if the system does not receive any reference pulses from the distributor, which means the engine is not running. Fuel cutoff also occurs at high engine RPM high vehicle speed or if a DTC 21, 22, 24 or 31 is set in order to protect the internal engine components from damage.
