PORT FUELINJECTION PRESSURE REGULATOR DIAGNOSIS

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PORT FUEL-INJECTION PRESSURE REGULATOR DIAGNOSIS
FIGURE 24-1 If the vacuum hose is removed from
the fuel-pressure regulator when the engine is
running, the fuel pressure should increase. If it
does not increase, then the fuel pump is not
capable of supplying adequate pressure or the
fuel-pressure regulator is defective. If gasoline
is visible in the vacuum hose, the regulator is
leaking and should be replaced.
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PRESSURE TRANSDUCER FUEL PRESSURE TEST
FIGURE 24-2 (a) A fuel-pressure graph after key
on, engine off (KOEO). (b) Pressure drop after 10
minutes on a normal port fuel-injection system.
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DIAGNOSING ELECTRONIC FUEL-INJECTION PROBLEMS
USING VISUAL INSPECTION

• All fuel-injection systems require the proper
  amount of clean fuel delivered to the system at
  the proper pressure and the correct amount of
  filtered air.
• The following items should be carefully inspected
  before proceeding to more detailed tests.
• Check the air filter and replace as needed.
• Check the air induction system for obstructions.
• Check the conditions of all vacuum hoses. Replace
  any hose that is split, soft (mushy), or brittle.
• Check the positive crankcase ventilation (PCV)
  valve for proper operation or replacement as
  needed
• Check all fuel-injection electrical connections
  for corrosion or damage.
• Check for gasoline at the vacuum port of the
  fuel-pressure regulator if the vehicle is so
  equipped.

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DIAGNOSING ELECTRONIC FUEL-INJECTION PROBLEMS
USING VISUAL INSPECTION
FIGURE 24-3 A clogged PCV system caused the
engine oil fumes to be drawn into the air cleaner
assembly. This is what the technician discovered
during a visual inspection.
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STETHOSCOPE FUELINJECTION TEST
FIGURE 24-4 All fuel injectors should make the
same sound with the engine running at idle speed.
A lack of sound could indicate an electrically
open injector, a break in the wiring, or a stuck
closed injector. A defective computer could also
be the cause of a lack of clicking (pulsing) of
the injectors.
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SCAN TOOL VACUUM LEAK DIAGNOSIS

• If a vacuum (air) leak occurs on an engine
  equipped with a speed-density-type of fuel
  injection, the extra air would cause the
  following to occur
• The idle speed increases due to the extra air
  just as if the throttle pedal was depressed.
• The MAP sensor reacts to the increased air from
  the vacuum leak as an additional load on the
  engine.
• The computer increases the injector pulse width
  slightly longer due to the signal from the MAP
  sensor.
• The air-fuel mixture remains unchanged.
• The idle air control (IAC) counts will decrease,
  thereby attempting to reduce the engine speed to
  the target idle speed stored in the computer
  memory.

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SCAN TOOL VACUUM LEAK DIAGNOSIS
FIGURE 24-6 Using a scan tool to check for IAC
counts or percentage as part of a diagnostic
routine.
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PORT FUEL-INJECTION SYSTEM DIAGNOSIS

• To determine if a port fuel-injection
  systemincluding the fuel pump, injectors, and
  fuel-pressure regulatorare operating correctly,
  take the following steps.
• Attach a fuel-pressure gauge to the Schrader
  valve on the fuel rail.
• Turn the ignition key on or start the engine to
  build up the fuel-pump pressure (to about 35 to
  45 PSI).
• Wait 20 minutes and observe the fuel pressure
  retained in the fuel rail and note the PSI
  reading.
• The check valve in the fuel pump
• Leaking injectors, lines, or fittings
• A defective (leaking) fuel-pressure regulator
• Reenergize the electric fuel pump.
• Clamp the fuel supply line, and wait 10 minutes
  (see caution box). If the pressure drop does not
  occur, replace the fuel pump. If the pressure
  drop still occurs, continue with the next step.

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PORT FUEL-INJECTION SYSTEM DIAGNOSIS
FIGURE 24-7 Checking the fuel pressure using a
fuel-pressure gauge connected to the Schrader
valve.
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PORT FUEL-INJECTION SYSTEM DIAGNOSIS

• To determine which unit is defective, perform the
  following
• Reenergize the electric fuel pump.
• Clamp the fuel supply line, and wait 10 minutes
  (see caution box). If the pressure drop does not
  occur, replace the fuel pump. If the pressure
  drop still occurs, continue with the next step.
• Repeat the pressure buildup of the electric pump
  and clamp the fuel return line.
• If the pressure drop still occurs, one or more of
  the injectors is leaking. Remove the injectors
  with the fuel rail and hold over paper. Replace
  those injectors that drip one or more drops after
  10 minutes with pressurized fuel.

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PORT FUEL-INJECTION SYSTEM DIAGNOSIS
FIGURE 24-8 Shutoff valves must be used on
vehicles equipped with plastic fuel lines to
isolate the cause of a pressure drop in the fuel
system.
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TESTING FOR AN INJECTOR PULSE

• One of the first checks that should be performed
  when diagnosing a no-start condition is whether
  the fuel injectors are being pulsed by the
  computer.
• Checking for proper pulsing of the injector is
  also important in diagnosing a weak or dead
  cylinder.

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TESTING FOR AN INJECTOR PULSE
FIGURE 24-9 (a) Noid lights are usually purchased
as an assortment so that one is available for any
type or size of injector wiring connector. (b)
The connector is unplugged from the injector and
a noid light is plugged into the injector
connector. The noid light should flash when the
engine is being cranked if the power circuit and
the pulsing to ground by the computer are
functioning okay.
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TESTING FOR AN INJECTOR PULSE

• Possible noid light problems and causes include
  the following
• The light is off and does not flash.
• The noid light flashes dimly.
• The noid light is on and does not flash.

FIGURE 24-10 Use a DMM set to read DC volts to
check the voltage drop of the positive circuit to
the fuel injector. A reading of 0.5 volt or less
is generally considered to be acceptable.
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CHECKING FUEL-INJECTOR RESISTANCE

• Each port fuel injector must deliver an equal
  amount of fuel or the engine will idle roughly or
  perform poorly.
• The electrical balance test involves measuring
  the injector coil-winding resistance.
• For best engine operation, all injectors should
  have the same electrical resistance.
• To measure the resistance, carefully release the
  locking feature of the connector and remove the
  connector from the injector.

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CHECKING FUEL-INJECTOR RESISTANCE
FIGURE 24-11 Connections and settings necessary
to measure fuel-injector resistance. (Courtesy of
Fluke Corporation)
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MEASURING RESISTANCE OF GROUPED INJECTORS

• To measure the resistance of these injectors, it
  is often easiest to measure each group of three
  that is wired in parallel.
• The resistance of three injectors wired in
  parallel is one-third of the resistance of each
  individual injector.
• For example,
• Injector resistance 12 ohms (O)
• Three injectors in parallel 4 ohms (O)

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MEASURING RESISTANCE OF INDIVIDUAL INJECTORS

• While there are many ways to check injectors, the
  first test is to measure the resistance of the
  coil inside and compare it to factory
  specifications.

FIGURE 24-14 If an injector has the specified
resistance, that does not mean that it is okay.
This injector had the specified resistance yet
did not deliver the specified amount of fuel
because it was clogged.
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PRESSURE-DROP BALANCE TEST

• The pressure balance test involves using an
  electrical timing device to pulse the fuel
  injectors on for a given amount of time, usually
  500 ms or 0.5 second, and observing the drop in
  pressure that accompanies the pulse.
• If the fuel flow through each injector is equal,
  the drop in pressure in the system will be equal.

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PRESSURE-DROP BALANCE TEST
FIGURE 24-15 Connect a fuel-pressure gauge to the
fuel rail at the Schrader valve.
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SCOPE TESTING FUEL INJECTORS

• A scope (analog or digital storage) can be
  connected into each injector circuit.
• There are three types of injector-driven drive
  circuits and each type of circuit has its own
  characteristic pattern.

FIGURE 24-17 A digital storage oscilloscope can
be easily connected to an injector by carefully
back probing the electrical connector.
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SCOPE TESTING FUEL INJECTORS

• Saturated Switch Type
• Peak-and-Hold Type
• Pulse-Width Modulated Type

FIGURE 24-18 The injector on-time is called the
pulse width. (Courtesy of Fluke Corporation)
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IF THREE OF SIX INJECTORS ARE DEFECTIVE, SHOULD I
ALSO REPLACE THE OTHER THREE?
FIGURE 24-20 A set of six reconditioned injectors.
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IDLE AIR SPEED CONTROL DIAGNOSIS

• On an engine equipped with fuel injection (TBI or
  port injection), the idle speed is controlled by
  increasing or decreasing the amount of air
  bypassing the throttle plate.
• Again, an electronic stepper motor or pulse-width
  modulated solenoid is used to maintain the
  correct idle speed.
• This control is often called the idle air control
  (IAC).

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IDLE AIR SPEED CONTROL DIAGNOSIS
FIGURE 24-21 An IAC controls idle speed by
controlling the amount of air that passes around
the throttle plate. More airflow results in a
higher idle speed.
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IDLE AIR SPEED CONTROL DIAGNOSIS
FIGURE 24-23 Some IAC units are purchased with
the housing as shown. Carbon buildup in these
passages can cause a rough or unstable idling or
stalling.
FIGURE 24-22 A typical IAC.
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THERE IS NO SUBSTITUTE FOR A THOROUGH VISUAL
INSPECTION
FIGURE 24-24 (a) Nothing looks unusual when the
hood is first opened. (b) When the cover is
removed from the top of the engine, a mouse or
some other animal nest is visible. The animal had
already eaten through a couple of injector wires.
At least the cause of the intermittent misfire
was discovered.
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FUEL-INJECTION SERVICE
FIGURE 24-26 Testing fuel-pump volume using a
fuel-pressure gauge with a bleed hose inserted
into a suitable container. The engine is running
during this test.
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FUEL-PUMP RELAY CIRCUIT DIAGNOSIS