Fuel Delivery and Air Induction: Description and Operation

Fuel Delivery System:

PURPOSE
The basic function of the fuel system is to control fuel delivery to the engine under all operating conditions.

GENERAL DESCRIPTION
The fuel system is controlled by the control module located in the passenger compartment. The control module is the control center of the system. Fuel is delivered to the engine by one type of fuel injection, throttle body fuel injection (TBI), and the injection is supplied fuel by the fuel supply system.
The main control sensor is the Oxygen Sensor (O2S), which is located in the exhaust manifold. The O2S tells the control module the amount of oxygen in the exhaust gas, and the control module changes 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. These parameters determine the "mode" of engine operation.

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

Starting Mode:
When the key is first turned ON, the control module turns ON the fuel pump relay for two seconds, and the fuel pump builds pressure up to the TBI unit. The control module checks the Engine Coolant Temperature (ECT) sensor, Throttle Position (TP) sensor, Manifold Absolute Pressure (MAP) sensor, and the Ignition signal, 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 control module 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 may be cleared by pushing the accelerator pedal down all the way. The control module then pulses the injector at a 16.5:1 air fuel ratio. The control module holds this injector rate as long as the throttle stays wide open, and the engine speed is below approximately 600 rpm. If the throttle position becomes less than approximately 65%, the control module returns to the starting mode.

Run Mode:
The run mode is the mode under which the engine operates most of the time. The run mode has two conditions, called OPEN LOOP and CLOSED LOOP.

OPEN LOOP:
When the engine is first started, and engine speed is above 400 rpm, the system goes into OPEN LOOP operation. In OPEN LOOP, the control module ignores the signal from the O2S, and calculates the air/fuel ratio based on inputs from the Engine Coolant Temperature (ECT) sensor and Manifold Absolute Pressure (MAP) sensor.

The system will stay in OPEN LOOP until the following conditions are met on non-heated O2S:
- The O2S has a varying voltage output, showing that it is hot enough to operate properly. (This depends on engine temperature)
- The ECT is above a specified temperature.
- A specified amount of time has elapsed after starting the engine.
A normal operating system may go into OPEN LOOP at an idle if the O2S temperature drops below the minimum requirement to produce voltage fluctuation.

CLOSED LOOP:
The specified values vary with different engines, and are stored in the Programmable Read Only Memory (PROM). When these conditions are met, the system goes into CLOSED LOOP operation. In CLOSED LOOP, the control module 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:
When the control module 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 control module observes a fast reduction in the throttle opening and a sharp decrease in manifold pressure, it causes the system to enter the deceleration mode by reducing the amount of fuel delivered to the engine. When deceleration is very fast, the control module 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 control module to operate in this mode, it first must sense the correct engine coolant temperature, ignition control, canister purge activity and constant engine speed. During Semi-CLOSED LOOP operation, there will be very little long term fuel trim (formerly known as Block Learn) and short term fuel trim (formerly known as Integrator), and the exhaust sensor values will read below 100 millivolts.

Decel En-Leanment:
On deceleration, the control module senses a high MAP vacuum (low voltage or kPa) and leans the fuel spray for emissions reasons, but it should be noted that the control module 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 control module 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 a control module system (With a Bi-directional scanner), and the transmission is in park, that the O2 sensor reading is low (usually below 100 mV) and the long term and short term fuel trim are both around the 128 counts, lower the engine speed to 1000 RPM. If the oxygen sensor and long term fuel trim numbers respond normally at this RPM, it's possible that the system was fooled into the decel en-leanment mode of operation. If the oxygen sensor and long term fuel trim 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 control module can compensate for the 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 Ignition (DI), which means the engine is not running. Fuel cut-off also occurs at high engine rpm, to protect internal engine components from damage. Vehicle speed also enables fuel cutoff.

CONSTRUCTION

Fuel System Components:

The fuel system consists of the following components.
- Throttle Body Injection (TBI) Unit.
- Fuel pump.
- Fuel pump relay circuit.
- Fuel tank.
- Accelerator control.
- Fuel lines.
- Fuel filter.
- Evaporative Emission (EVAP) control system.

BASIC SYSTEM OPERATION:
The fuel supply is stored in the fuel tank. An electric fuel pump, located in the fuel tank with the fuel level gauge sending unit, pumps fuel through an in-line filter to the TBI unit. The pump is designed to provide fuel at a pressure greater than is needed by the injectors. The pressure regulator, part of the TBI assembly, keeps fuel available to the injectors at a regulated pressure. Unused fuel is returned to the fuel tank by a separate line.

Throttle Body Injection (TBI) Unit:
Throttle Body Injection (TBI) Unit consists of three assemblies:
- Fuel meter cover with:
- Fuel pressure regulator.
- Fuel meter body with:
- Fuel injectors.
- Throttle body with:
- Idle air control valve.
- Throttle position sensor.

Fuel Pump:
On 4.3L, 5.0L and 5,7L a pressure regulator in the TBI keeps fuel available to the injectors at a constant pressure between 62 and 90 kPa (9 to 13 psi). The pump is designed to provide pressurized fuel at about 125 kPa (18 psi).
On 7.4L TBI applications regulated fuel pressure is 179 to 220 kPa (26-32 psi).