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5B.3
Section 5B
EFI Fuel System
5B
EFI Fuel System Components
General
The Electronic Fuel Injection (EFI) system is a
complete engine fuel and ignition management design.
The system includes the following principal
components:
• Fuel Pump
• Fuel Filter
• Fuel Rail
• Fuel Line(s)
• Fuel Pressure Regulator
• Fuel Injectors
• Throttle Body/Intake Manifold
• Engine Control Unit (ECU)
• Ignition Coils
• Engine (Oil) Temperature Sensor
• Throttle Position Sensor (TPS)
• Speed Sensor
• Oxygen Sensor
• Wire Harness Assembly & Affiliated Wiring,
• Malfunction Indicator Light (MIL)
Operation
The EFI system is designed to provide peak engine
performance with optimum fuel efficiency and lowest
possible emissions. The ignition and injection functions
are electronically controlled, monitored and continually
corrected during operation to maintain the theoretical
ideal or “stoichiometric” air/fuel ratio of 14.7:1.
The central component of the system is the Motronic
™
Engine Control Unit (ECU) which manages system
operation, determining the best combination of fuel
mixture and ignition timing for the current operating
conditions.
An electric fuel pump is used to move fuel from the
tank through the fuel line and in-line fuel filter. A fuel
pressure regulator maintains a system operating
pressure of 39 psi and returns any excess fuel to the
tank. At the engine, fuel is fed through the fuel rail and
into the injectors, which inject it into the intake ports.
The ECU controls the amount of fuel by varying the
length of time that the injectors are “on.” This can
range from 1.5-8.0 milliseconds depending on fuel
requirements. The controlled injection of the fuel
occurs each crankshaft revolution, or twice for each
4-stroke cycle. One-half the total amount of fuel
needed for one firing of a cylinder is injected during
each injection. When the intake valve opens, the fuel/
air mixture is drawn into the combustion chamber,
ignited, and burned.
The ECU controls the amount of fuel injected and the
ignition timing by monitoring the primary sensor signals
for engine temperature, speed (RPM), and throttle
position (load). These primary signals are compared to
preprogrammed “maps” in the ECU computer chip,
and the ECU adjusts the fuel delivery to match the
mapped values. An oxygen sensor provides continual
feedback to the ECU based upon the amount of
unused oxygen in the exhaust, indicating whether the
fuel mixture being delivered is rich or lean. Based upon
this feedback, the ECU further adjusts fuel input to
reestablish the ideal air/fuel ratio. This operating mode
is referred to as “closed loop” operation. The EFI
system operates “closed loop” when all three of the
following conditions are met:
a. The oil temperature is greater than 35°C (95°F).
b. The oxygen sensor has warmed sufficiently to
provide a signal (minimum 375°C, 709°F).
c. Engine operation is at a steady state (not starting,
warming up, accelerating, etc.).
During “closed loop” operation the ECU has the ability
to readjust temporary and learned adaptive controls,
providing compensation for changes in overall engine
condition and operating environment, so it will be able
to maintain the ideal air/fuel ratio of 14.7:1. The system
requires a minimum engine oil temperature greater
than 55°C (130°F) to properly adapt. These adaptive
values are maintained as long as the ECU is “powered
up” by the battery.
During certain operating periods such as cold starts,
warm up, acceleration, etc., an air/fuel ratio richer than
14.7:1 is required and the system operates in an “open
loop” mode. In “open loop” operation the monitoring of
exhaust gases (output) is not used, and the controlling
adjustments are based on the primary sensor signals
and programmed maps only. The system operates
“open loop” whenever the three conditions for closed
loop operation (above) are not being met.