Fuel Delivery and Air Induction: Description and Operation
GENERAL DESCRIPTION - TBI FUEL SYSTEM
Computerized Engine Control.:
Fuel Flow And Control.:
PURPOSE
The purpose of the fuel control system is to deliver the correct amount of fuel to the engine. Fuel is delivered to the engine by a throttle body injection (TBI) unit.
The powertrain control module (PCM) controls the amount of fuel delivered by changing how long the injector is turned "ON" and "OFF". This is done by pulsing the injector for very short times.
FUEL SYSTEM DESCRIPTION
The SATURN fuel system is comprised of a polyethylene fuel storage tank. This tank is equipped with a submersible electric fuel pump, reservoir, and fuel level sending unit module attached at the top of the fuel tank. This module is held in place with a cam lock ring and sealed with a rubber O-ring seal. Fuel is drained from the tank by removing the filler tube at tank and pushing the check ball into the tank.
Fuel enters the fuel storage tank through a lead tin plated steel filler neck attached to the tank with a short section of rubber hose. This system is sealed from the environment with a fuel filler cap.
Fuel is delivered from the pump to the engine and from the engine back to the tank by way of nylon fuel lines with quick connect line connections. Before entering the engine, fuel passes through an in-line fuel filter also equipped with quick connects. Fuel vapor generated within the fuel storage tank is stored in a charcoal vapor canister where it is later purged and consumed by the engine.
MODES OF OPERATION
Fuel delivery, controlled by the powertrain control module (PCM), is based on several important engine operation parameters, which also determined the mode of engine operation. These parameters include engine speed, manifold pressure, engine coolant temperature, throttle position, and exhaust oxygen content.
STARTING MODE
When the key is first turned to the "ON" position, the PCM will turn "ON" the fuel pump relay for two seconds allowing the fuel pump to build pressure to the TBI unit. The PCM then monitors the coolant temperature, throttle position, manifold pressure, and ignition signal to determine the proper fuel for starting. This ranges from 1.5:1 at -36°C (-33°F) to 14.6:1 at 94°C (201°F).
CLEAR FLOOD MODE
If the engine floods, it can be cleared by pressing the accelerator pedal all the way to the floor. The PCM turns "OFF" the injector. The PCM holds this injector rate as long as the throttle stays wide open and the engine speed is below 400 rpm. If the throttle position becomes less than 80 percent, the PCM returns to the starting mode.
RUN MODE - OPEN LOOP
When the engine is first started, and engine speed is above 400 RPM, the system goes into Open Loop operation. The PCM ignores the signal from the oxygen sensor, and calculates the fuel required based on inputs from the coolant temperature and manifold absolute pressure (MAP) sensors. The system stays in open loop until the following conditions are met:
^ The oxygen sensor has varying voltage output, showing that it is hot enough to operate properly depending on temperature.
^ The coolant temperature sensor is above a specified temperature.
^ A specific amount of time has elapsed after starting the engine.
RUN MODE - CLOSED LOOP
Specific values for the above conditions exist and are stored in the memory calibration module. When these values are met, the system goes into Closed Loop operation. In closed loop, the PCM calculates the fuel required in the same manner as open loop but corrects for small errors based on the signal from the oxygen sensor. The best mixture to minimize exhaust emissions is 14.7:1 air/fuel ratio, which allows the catalytic converter to operate most efficiently. Because of the constant measuring and adjusting of the air/fuel ratio, the injection system is called a closed loop system.
ACCELERATION MODE
The PCM responds to rapid increases in throttle position and manifold pressure (MAP), providing extra fuel needed for smooth acceleration.
DECELERATION MODE
The PCM responds to decreases in throttle position and manifold pressure (MAP) reduces the amount of fuel. When deceleration is extended, the PCM cuts "OFF" fuel completely when power is not needed.
BATTERY VOLTAGE CORRECTION MODE
When battery voltage is low, the PCM compensates by:
^ Increasing the injector pulse width to provide proper fuel.
^ Increasing the idle rpm to increase generator output.
FUEL CUT-OFF MODE
To prevent possible engine damage from overspeed, the PCM cuts "OFF" fuel from the injectors when engine speed is above 6750 rpm in drive or while vehicle is moving. In neutral or while stopped, the engine speed is limited to 4000 rpm.
FUEL SYSTEM COMPONENTS
The fuel system consists of the following parts:
1. Fuel Tank, Lines and Pump
2. Fuel Pump Relay and Electrical Circuit
3. Throttle Body Injection (TBI) Unit
a. Throttle Body
b. Fuel Injector
c. Fuel Pressure Regulator
d. Idle Air Control (IAC) Valve
BASIC SYSTEM OPERATION
The fuel is delivered to the TBI unit and circulated through the system by an in-tank fuel pump. The fuel gage sending unit is mounted on the side of the pump module. Fuel is pumped through a fuel supply line to an in-line fuel filter and the TBI assembly. A pressure regulator in the TBI keeps the fuel pressure at the injector between 179-214 kPa (26-31 psi). Excess fuel, not needed by the injector, is returned to the fuel tank by a separate line. In order to properly control the fuel supply, the fuel pump is controlled by the PCM through the fuel pump relay.
FUEL SUPPLY COMPONENTS
The fuel supply is stored in the fuel tank. An electric high pressure fuel pump, located in the fuel tank with the gage sending unit, pumps fuel through an in-line fuel filter to the TBI assembly. The pressure regulator, part of the TBI assembly, keeps fuel available to the injector at a controlled pressure. Unused fuel is returned to the fuel tank by a separate line. Nylon fuel supply and return lines are routed along the driver side of the vehicle from the fuel tank. Quick connects have been installed at the tank, fuel filter and engine for ease of assembly and serviceability.
FUEL PUMP ELECTRICAL CIRCUIT
When the key is first turned to the "ON" position, without the engine running, the PCM turns the fuel pump relay "ON" for two seconds. This builds up the fuel pressure quickly. If the engine is not started within two seconds, the PCM shuts the fuel pump "OFF" and waits until ignition reference pulses are present. As soon as the engine is cranked, the PCM turns the relay "ON" which powers the fuel pump.