General System Description
Fuel Control Description for CPI (Central Port Injection)
The fuel control system is controlled by an Electronic Control Module (ECM) located in the passenger compartment. The basic function of the fuel control system is to control fuel delivery to the engine. Fuel is delivered to the engine by a Central Port Injection (CPI) unit. The main control sensor is the Oxygen (O2) sensor, which is located in the exhaust manifold. The O2 sensor tells the ECM the amount of oxygen in the exhaust gas, and the ECM changes the air/fuel ratio to the engine by controlling the fuel injector. A 14.7:1 air/fuel ratio is required for efficient catalytic converter operation. Because the constant measuring and adjusting of the air/fuel ratio, the fuel injection system is called a "Closed Loop" system. Several other important engine operation parameters include: engine speed, manifold pressure, engine coolant temperature, and throttle position. These parameters determine the "mode" of engine operation.
Fuel Control Operation Components
The fuel control system consists of the following components:
- Central Port Injection (CPI)
- Fuel pump
- Fuel pump relay
- Fuel tank
- Accelerator control components
- Fuel lines
- Fuel filter
- Evaporative emission control system
Operation
The fuel control system has an electric fuel pump. located in the fuel tank on the gage sending unit. It pumps fuel to the CPI unit through an in-line fuel filter and fuel supply line. The pump provides fuel at a pressure above the regulated pressure needed by CPI injector. A pressure regulator in the CPI unit keeps fuel available to the injector at a constant pressure. Fuel in excess of injector needs is returned to the fuel tank by a separate line. The ECM controls the injector that is located in the Central Port Injection (CPI) assembly. The CPI delivers fuel in response to the pulse width mandated signal from the ECM in one of several modes. In order to properly control the fuel supply, the fuel pump is operated by the ECM through the fuel pump relay and oil pressure switch.
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 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 CPI unit. The ECM checks the Coolant Temperature Sensor (CTS), Intake Air Temperature (IAT) sensor, Throttle Position Sensor (TPS), and Manifold Absolute Pressure (MAP) sensor, 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 time.
Clear Flood Mode
If the engine floods, clear it by pushing the accelerator pedal down all the way. The ECM when pulses the injector at 16.5:1 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 65%, the ECM returns to the starting mode.
Run Mode
The Run Mode is the mode under which the engine operates most of the time. In this mode, the engine operates in one of two conditions: "Open Loop" and "Closed Loop."
Open Loop/Closed 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 O2 sensor, and calculates the air/fuel ratio based on inputs from the Coolant Temperature Sensor (CTS) and Manifold Absolute Pressure (MAP) sensor. The system initiates in "Open Loop" while the ignition is turned to the "ON" position and battery voltage is applied to the Oxygen sensor's built-in heater element. Approximately 30 seconds after voltage is applied to the heater, the sensor reaches a temperature above 500°C, then "Closed Loop" operation is achieved. Throughout various vehicle operational conditions, the heater element ensures the sensor consistently operates in that temperature range. Heated sensors contain an internal, electrically powered heater. As such, they are not as dependent on engine heat to achieve minimum operation temperature. The heated sensor becomes active faster during engine warm up than an unheated sensor. This may or may not be desirable, depending on the design of the particular emission control system. The heated sensor remains active during long idles on engines with cool exhaust while the unheated sensor may cool below its minimum operating point and stop satisfactory function. This allows the air/fuel ratio to stay close to 14.7:1.
Acceleration Mode
When the ECM senses rapid changes in throttle position and manifold pressure, the system enters the Acceleration Mode and provides the extra fuel needed for smooth acceleration.
Deceleration Mode
When deceleration occurs, the fuel remaining in the intake manifold can cause excessive emissions and backfiring. When the ECM observes a fast reduction in throttle opening and a sharp decrease in manifold pressure, it causes the system to enter the Deceleration Mode, reducing the amount of fuel delivered to the engine. When deceleration is very fast, the ECM cuts off fuel completely for short periods.
Highway Fuel Mode (Semi-"Closed Loop")
This mode comes into operation at highway speeds and it's purpose is to improve fuel economy. For the ECM to operate in this mode, it first must sense the correct engine temperature, spark timing, canister purge activity and constant engine speed. During semi-"Closed Loop" operation, there will be very little block learn and integrator movement and the oxygen sensor values will read below 100 millivolts.
Decel En-leanment
On deceleration, the ECM senses a high MAP vacuum (low voltage or kPa) and leans the fuel spray for emission reasons, but it should be noted that the ECM can trigger this condition (Decel En-leanment) while the vehicle is not moving.
Decel En-leanment Operation
This mode of operation can be mis-diagnosed as a lean condition. The ECM will run the system lean on decel, or if the MAP sensor senses a low voltage (high engine vacuum), with the vehicle standing still, it will lean out the fuel delivery. If it is noted while testing an ECM system and the transmission is in park, that the O2 reading is low (usually below 100 mV) and the block learn and integrator are both around 128 counts, lower the engine speed to 1000 rpm. If the oxygen sensor and block learn numbers respond normally at this rpm, it is possible that the system was fooled into the Decel En-leanment mode of operation. If the oxygen sensor and block numbers do not respond at the lower rpm readings, there are other problems with the vehicle.
Battery Voltage Correction Mode
When battery voltage is low, the ECM can compensate for a weak spark delivered to the distributor by:
- Increasing injector on time
- Increasing the idle rpm
- Increasing ignition dwell time
Fuel Cut-Off Mode
No fuel is delivered by the injector 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. Fuel cut-off also occurs at high engine rpm, to protect internal engine components from damage.