Chapter
C
: Fuel, Air Induction, and Exhaust Systems Diagnosis and Repair
58
bleshooting a fuel injection problem. Special pressure
gauges are required for checking both low-pressure
throttle body systems and high-pressure multiport sys-
tems. Most throttle body injection systems operate at
about 10 to 15 psi (69 to 103 kPa) of pressure. Pressure
in a port injection system can vary from about 35 psi
(240 kPa) to over 60 psi (414 kPa). Accurate specifica-
tions and procedures for the system being tested, along
with a test gauge that indicates the proper pressure
range, are required.
To provide quick start-up, all injection systems retain
pressure when the engine is off. Therefore, fuel system
pressure must be relieved before opening fittings to
connect the gauge.
Relieving Fuel Pressure
There are several methods of relieving fuel pressure.
Follow the Service Manual recommendations for the
system being tested. Typical procedures are:
• Apply vacuum to the fuel pressure regulator using
a hand-vacuum pump, figure 3-8
• Use a special pressure gauge that has a relief valve,
figure 3-9. Attach the gauge to a Schrader valve on
the throttle body or fuel rail. Then press the button
on the gauge to release pressure Fig. 3-9. Fuel pres-
sure gauge with relief value for servicing Schrader
valve-equipped fuel injection systems.
• Remove the fuel pump fuse and start the engine.
Allow the engine to run until it dies
Fuel Injection Pressure Test
The fuel pumps used with fuel injection systems are ca-
pable of producing fuel pressure and delivery volume far
greater than required by the engine. For this reason, the
pressure and volume tests for these pumps measure
values other than actual pump output. Typically, speci-
fications are provided for checking the system pressure,
which is the regulated pressure supplied to the injectors.
System pressure will vary depending upon system de-
sign, as well as engine temperature and other operating
conditions.
To test system pressure on throttle body injection sys-
tems, tee a pressure gauge at the throttle body fuel de-
livery line, or attach the gauge to the test-port Schrader-
valve. Some port injection systems also have a
Schrader valve on the fuel distribution rail for attaching
the gauge. If not, the gauge installs into a fuel line with
a tee fitting, banjo fitting, or other adapter. Install the
gauge, start the engine, and allow it to run at idle. Check
the gauge reading and make sure the pressure is within
specifications. Be sure to check pressure at all speeds,
temperatures, and operating conditions specified by the
manufacturer. Low system pressure readings may be
caused by a faulty pump, a defective pressure relief
valve, or a clogged fuel filter.
To check rest, or residual pressure, shut the engine off
and note the gauge reading. Leave the gauges at-
tached, wait the recommended time, usually 10 to 20
minutes, and take another gauge reading. For most sys-
tems, residual pressure should not drop below about 15
psi (103 kPa).
DISCONNECTED
VACUUM HOSE
VACUUM
PUMP
FUEL PRESSURE
REGULATOR
PRESSURE
RELEASE
BUTTON
PRESSURE
GAUGE
SCHRADER VALVE
FITTING
FUEL DRAIN
TUBE
Fig. 3-8.
Relieving fuel pressure by applying vacuum to the pressure
regulator.
Fig. 3-9.
Fuel pressure gauge with relief value for servicing Schrader
valve-equipped fuel injection systems.
Summary of Contents for ASE-A8
Page 2: ......