Chapter
C
: Fuel, Air Induction, and Exhaust Systems Diagnosis and Repair
60
External Electric Pump
Older fuel injected vehicles use an external electric fuel
pump located underneath the vehicle on a frame rail
near the fuel tank. Most vehicles have a protective cover
over the pump housing which must be removed to ac-
cess the pump fittings and fasteners. To remove the
pump, disconnect the electrical wiring and fuel lines,
then unbolt the pump and lift it from the frame rail. To in-
stall the pump, loosely fit the fuel lines, bolt the pump to
the frame rail, then tighten the fuel lines and make the
electrical connections.
Fuel Pressure Regulator Service
Fuel injection systems all use some form of pressure
regulator to maintain uniform fuel pressure at the injec-
tors. A pressure regulator that is out of specifications
can cause hard starting and numerous performance
problems. Make sure there are no fuel supply problems
or line restrictions before replacing the pressure regula-
tor.
Return-Type Fuel Systems
The fuel pressure regulator contains a pressure cham-
ber separated by a diaphragm relief valve assembly with
a calibrated spring in the vacuum chamber side, figure
3-11. Fuel pressure is regulated when pump pressure,
acting on the bottom side of the diaphragm, overcomes
the force of the spring action on the top side. The di-
aphragm relief valve moves, opening or closing an ori-
fice in the fuel chamber to control the amount of fuel re-
turned to the fuel tank.
Vacuum acting on the top side of the diaphragm along
with spring pressure controls fuel pressure. A decrease
in vacuum creates an increase in fuel pressure. An in-
crease in vacuum creates a decrease in fuel pressure.
Example: Under heavy load conditions, the engine re-
quires more fuel flow. Vacuum decreases under heavy
load conditions because of throttle opening. A decrease
in vacuum allows more pressure to the top side of the
pressure relief valve, thus increasing fuel pressure.
Returnless Fuel Systems
Many late-model vehicles use a returnless fuel system.
This system regulates fuel pressure utilizing a mechan-
ical regulator without the vacuum assist. The regulator
can be mounted anywhere in the fuel pressure line and
many are located in the fuel tank connected to the in-
tank pump housing, figure 3-12.
These systems rely on the PCM to control fuel flow from
the injectors into the engine based on the input from en-
gine sensors. See Chapter Five of this book for more in-
formation on computer controlled engine management
systems.
FUEL INJECTION CONTROL SYSTEMS
Electronic fuel injection systems use sensors to monitor
operating conditions and actuators to adjust the airfuel
ratio to provide cold enrichment, acceleration enrich-
ment, deceleration fuel shut-off, and idle speed stabi-
Fig. 3-11.
Fuel pressure regulator operation.
Summary of Contents for ASE-A8
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