Preliminary --If
the engine runs but displays driveability symptoms that may or
may not be fuel-related (hard starting, hesitation, misfiring,
power loss), first atempt to eliminate other possible causes
of the problem.
1.Check the vehicle's on-board diagnostic system; correct any
fault codes.
2.Check the ignition system: spark plugs, wires, coil,
timing. Observe ignition patterns on a scope to identify ignition and
fuel problems.
3.Check for vacuum leaks at hoses, throttle body, intake
manifold, power brake booster, vacuum-powered accessories.
4.Check
the EGR and PCV systems. A leaky EGR valve can imitate a vacuum leak,
leaning out the air/fuel mixture. A clogged PVC valve or hose can cause
a rich mixture, while an air leak may create a lean condition.
5.Run
a power balance test to reveal weak cylinders, a power loss caused
by leaky valves or clogged injectors.
Electrical --Check the fuse first. If it's blown,
the most likely reason is a hot short in the relay, or the wiring
between the relay and the pump. Next, check other electrical components
using a voltmeter and wiring diagram.
Especially make sure the pump
relay is receiving the correct voltage from the control module
and is sending the correct voltage to the fuel pump. Attach voltmeter
leads to the hot and ground terminals, turn the key on and check
for correct voltage. Turn the key off, move the hot lead to the
relay's output terminal, and turn the key on. The relay should
click, and the voltmeter should read the specified voltage. You
have only seconds to check this before the ECM shuts down the relay.
If nothing happens (no click, no needle), test voltage output at
the relay driver terminal on the engine control module. A voltage
signal here shows the ECM is supplying power, but the relay isn't
responding. (No voltage at the ECM would indicate a defective driver
circuit.) The final check is continuity in the wiring between the
ECM and the relay. If it's okay, replace the relay.
To isolate an electrical problem other than the relay, bypass
other components one at a time using a jumper wire. If the jumper
restores power to the fuel pump, replace the bypassed electrical
component.
Pressure --If the engine runs but exhibits driveability
problems that suggest fuel delivery problems (too much or too little),
the only way to know for sure is by checking fuel-system pressures.
Hook up a pressure gauge, start the engine, and compare readings
to the manufacturer's specs.
Usually the operating pressure specification
is at idle with the vacuum hose attached to the regulator.Maximum
system pressure (wide-open throttle operation) is read with the
hose disconnected or pinched off.
If the engine hesitates when accelerating,
monitor fuel pressure during a test drive by fitting an EFI-approved
hose (200 PSI) long enough to reach the pressure gauge wherever
it can be observed from the driver's seat.
Read "deadhead" or maximum
pump pressure (higher than maximum fuel-system operating pressure)
by momentarily pinching off the return line while the engine is
running, or by connecting the gauge directly to the fuel line (no
tee).
Check the system's ability to hold pressure after shutdown
by turning off the key and watching the gauge. If the pressure
drops suddenly, the pump check valve may be stuck open. If the
pressure bleeds off slowly, it may indicate a leaky check valve/fuel
line/ fuel injector, or a faulty regulator.
Too High . The mostly likely cause is a faulty regulator
or a clogged return line. Disconnect the return line at the regulator,
install another hose and reroute the return fuel to an appropriate
container. Start the engine and read the fuel pressure. If it's
now within specifications, check for a blocked or kinked return
line or a clogged or stuck roller valve in the line.
If fuel pressure
is still too high with the return line disconnected, check the
fuel volume flowing through the bypass hose. If it's little or
none, pull a vacuum on the regulator with a hand pump. No increase
indicates a faulty regulator. But if the flow increases, and the
system pressure comes back down to spec, look for a leaky, blocked
or misrouted vacuum line.
T oo Low. The many possibilities include: a clogged
in-tank pump strainer; a plugged fuel filter; a blocked or kinked
fuel line; a leaky pulsator, fuel line or hose; a leaky pump check
valve; a defective fuel-pressure regulator; or simply the wrong
fuel pump for the application.
Check for filter and fuel line blockages
by teeing a gauge into the tank outlet line and reading fuel pressure
there. A number at least several PSI higher than at the engine
indicates a restricted filter or fuel line. If there's no difference
in pressure between outlet and engine, the pump is weak or its
strainer is clogged. Either way, you must pull to pump to find
out.
Causes of low pump output include low voltage, a bad ground
wire and a leaky or defective pump check valve. Usually the check
valve is integral to the pump. If it's faulty you must replace
the pump. High levels of alcohol in fuel can cause rubber check
balls to swell and stick. A leaking coupling hose or the pump's
pulsator also may cause low pressure in the system.
Flow.Some manufacturer's specs for checking
pump output require measuring the volume of fuel delivered at idle
over time. Do this quickly and safely by disconnecting the return
line at the regulator and installing a bypass to a safe container.
Check the system's pressure at the same time.
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