Modes of Operation

MODES OF OPERATION
The ECM monitors voltages from several sensors to determine how much fuel to give the engine. The fuel is delivered under one of several conditions, called "modes". All the modes are controlled by the ECM.

STARTING MODE
When the ignition switch is turned to the "ON" position (before engaging starter), the ECM turns "ON" the FP relay for two seconds, and the FP builds up pressure to the TBI unit. The ECM checks the Engine Coolant Temperature Sensor, TP Sensor, Manifold Absolute Pressure sensor and crank signal, then determined 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) Engine Coolant Temperature (ECT).
The Engine Control Module (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 times.

CLEAR FLOOD MODE
If the engine floods, it can be cleared by pushing the accelerator pedal down all the way. The ECM then pulses the injector at a 20:1 air/fuel ratio, and 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%, the ECM returns to the starting mode.

RUN MODE
The run mode has three conditions:

Open Loop
When the engine is first started, and it is above 400 rpm, the system goes into "Open Loop" operation. In "Open Loop", the ECM ignores the signal from the Oxygen Sensor (O2S), and calculates the air/ fuel ratio based on inputs from the coolant temperature and Manifold Absolute Pressure sensors.
The system stays in "Open Loop" until the following conditions are met:
1. The O2S has varying voltage output, showing that it is hot enough to operate properly. (This depends on temperature.)
2. The ECT Sensor is above a specified temperature.
3. A specific amount of time has elapsed after starting the engine.
4. The throttle is above idle.

Closed Loop
The specific values for the above conditions vary with different engines, and are stored in the Programmable Read Only Memory (PROM). When these values are met, the system goes into "Closed Loop" operation. In "Closed Loop", the ECM calculates the air/fuel ratio (injector on-time) based on the signal from the 02S. This allows the air/fuel ratio to stay very close to 14.7:1.

Highway Mode (Semi- Closed Loop)
When the vehicle is driven at highway speeds, the system may enter "Highway" or "Semi-Closed Loop" mode for improved fuel economy. For the system to enter the "Highway" mode, the ECM must first see the correct engine temperature, spark timing, Evaporative emission canister purge activity, and constant vehicle speed.
While operating in this mode, the air/fuel mixture may be leaned out somewhat for optimum fuel economy.
During "Highway" mode operation, the ECM will periodically switch back to "Closed Loop" mode to allow a check of the engine systems. If all systems meet the requirements, the ECM will switch to the "Highway" mode again.
Using a "Scan" tool, it is possible to determine when the system is operating in "Highway" mode by looking at integrator/block learn values and O2S voltage:
- Integrator and block learn values will show very little change.
- O2S voltage, normally between 0 and 1000 millivolts, will be less than 100 millivolts.

ACCELERATION MODE
The ECM looks at rapid changes in throttle position and manifold pressure, 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 throttle position and manifold pressure and reduces the amount of fuel. When deceleration is very fast, the ECM can cut off fuel completely for short periods of time.

BATTERY VOLTAGE CORRECTION MODE
When battery voltage is low, the ECM can compensate for a weak spark delivered by the distributor by:
- Increasing injector "ON" time of fuel delivered;
- Increasing the idle rpm.

FUEL CUTOFF MODE
No fuel is delivered by the injectors when the ignition is "OFF". This prevents dieseling. Also, fuel is not delivered if no reference pulses are seen from the distributor, which means the engine is not running.