chap60 pps Automotive technology at University of Cambridge

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Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
By James D. Halderman

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OBJECTIVES:
After studying Chapter 60, the reader should
be able to:

•

•
•

Prepare for ASE Engine Performance (A8)
certification test content area “C” (Fuel, Air

Induction, and Exhaust Systems Diagnosis
and Repair).
Describe how to check an electric fuel pump
for proper pressure and volume delivery.
Explain how to check a fuel-pressure
regulator.
Continued

Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
By James D. Halderman

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OBJECTIVES:
After studying Chapter 60, the reader should
be able to:

•
•
•

Describe how to test fuel injectors.
Explain how to diagnose electronic fuel
injection problems.
Describe how to service the fuel injection
system.

Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
By James D. Halderman

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2009Pearson
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KEY TERMS:
graphing multimeter (GMM)
IAC counts
noid light
peak-and-hold injector • pressure transducer
saturation

Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
By James D. Halderman

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PORT FUEL-INJECTION PRESSURE
REGULATOR DIAGNOSIS
Most port­fuel­injected engines use a vacuum hose connected to 
the fuel­pressure regulator. At idle, the pressure inside the intake 

manifold is low (high vacuum).
Manifold vacuum is applied above the diaphragm inside the fuel­
pressure regulator. This reduces the pressure exerted on the 
diaphragm and results in a lower, about 10 psi (69 kPa), fuel 
pressure applied to the injectors. 

Continued
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To test a vacuum­controlled fuel­pressure regulator:
1. Connect a fuel­pressure gauge to monitor the fuel pressure.
2. Locate the fuel­pressure regulator and disconnect the vacuum 
hose from the regulator.

3. With the engine running at idle speed, reconnect the vacuum 
hose to the fuel­pressure regulator while watching the fuel­
pressure gauge. The fuel pressure should drop (about 10 psi 
or 69 kPa) when the hose is reattached to the regulator.
NOTE: If gasoline drips out of the vacuum hose when removed from
the fuel­pressure regulator, the regulator is defective and will require
replacement.

Continued
Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
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4. Using a hand­operated vacuum pump, apply vacuum (20 in. 
Hg) to the regulator. The regulator should hold vacuum. If 

the vacuum drops, replace the fuel­pressure regulator.
NOTE: Some vehicles do not use a 

vacuum­regulated fuel­pressure regulator. 
Many of these vehicles use a regulator 
located inside the fuel tank that supplies a 
constant fuel pressure to the fuel injectors.
Figure 60–1
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.
Continued
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Pressure Transducer Fuel Pressure Test
Using a pressure transducer and a graphing multimeter (GMM) or digital
storage oscilloscope (DSO) allows the service tech to view fuel pressure
over time, Figure a. Note the fuel pressure dropped from 15 to 6 psi on a
TBI-equipped vehicle after one minute. Normal pressure holding capability is
shown in Figure b, when pressure dropped about 10% after 10 minutes.

(a)

(b)

Figure 60–2 (a) A fuel-pressure graph after key on, engine off (KOEO). (b) Pressure drop after 10
minutes on a normal port fuel-injection system.
Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
By James D. Halderman

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DIAGNOSING ELECTRONIC FUEL-INJECTION
PROBLEMS USING VISUAL INSPECTION
All fuel­injection 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 all fuel­injection electrical connections for corrosion 
or damage.
Continued
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NOTE: Use of an incorrect PCV valve can cause a rough idle or stalling.
Figure 60–3 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.

Check the positive crank­
case ventilation (PCV)
valve for proper operation
or replacement as needed. 
Check for gasoline at the vacuum port of the fuel­pressure 
regulator if the vehicle is so equipped. Gasoline in the vacuum 
hose at the fuel­pressure regulator indicates that the regulator is 
defective and requires replacement.
Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
By James D. Halderman

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Stethoscope Fuel Injection Test - Part 1
A common test for injector operation is to listen to the injector using a
stethoscope with the engine at idle speed. All injectors should produce the
same clicking sound. If an injector makes a clunking or rattling sound, it
should be tested further or replaced.

Figure 60–4
All fuel injectors should make the same sound
with the engine running at idle speed. A lack of
sound indicates a possible electrically open
injector or a break in the wiring. A defective
computer could also be the cause of a lack of
clicking (pulsing) of the injectors.

Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
By James D. Halderman

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Stethoscope Fuel Injection Test - Part 2
With the engine still running, place the end of the stethoscope probe to the
return line from the fuel-pressure regulator. Fuel should be heard flowing
back to the fuel tank if the fuel-pump pressure is higher than the fuelregulator pressure. If no sound of fuel is heard, then either the fuel pump or
the fuel-pressure regulator is at fault.

Figure 60–5 Fuel should be heard returning to the
fuel tank at the fuel return line if the fuel-pump
and fuel-pressure regulator are functioning
correctly.

Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
By James D. Halderman

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Quick and Easy Leaking Injector Test
Leaking injectors may be found by disabling the ignition, unhooking all
injectors, and checking exhaust for hydrocarbons (HC) using a gas
analyzer while cranking the engine (maximum HC = 300 ppm).

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SCAN TOOL VACUUM LEAK DIAGNOSIS
If a vacuum (air) leak occurs on an engine equipped with a speed­
density­type of fuel injection, the extra air would cause the 
following to occur:
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.
Idle air control (IAC) counts will decrease, thereby attempting 
to reduce the engine speed to the target idle speed stored in the 
computer memory. See Figure 60–6.
Continued
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Figure 60–6
Using a scan tool to check for IAC counts or percentage as part of a diagnostic routine.

Continued
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One of the best indicators of a vacuum leak on a speed­density fuel­
injection system is to look at the IAC counts or percentage, usually 
15 to 25. A reading of less than 5 indicates a vacuum leak.
If a vacuum leak occurs on an engine equipped with a mass airflow­
type fuel­injection system, the extra air causes the following:
The engine will operate leaner­than­normal because the extra 
air has not been measured by the MAF sensor.
The idle speed will likely be lower due to the leaner­than­
normal air–fuel mixture.
The idle air control (IAC) counts or percentage will often 
increase in an attempt to return the engine speed to the target 
speed stored in the computer.
Continued
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No Spark, No Squirt
Most electronic fuel-injection computer systems use the ignition primary
(pickup coil or crank sensor) pulse as the trigger for when to inject (squirt)
fuel from the injectors (nozzles). If this signal were not present, no fuel
would be injected. Because this pulse is also necessary to trigger the
module to create a spark from the coil, it can be said that “no spark” could
also mean “no squirt.” If the cause of a no-start condition is observed to be
a lack of fuel injection, do not start testing or replacing fuel-system
components until the ignition system is checked for proper operation.

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PORT FUEL INJECTION SYSTEM DIAGNOSIS
To determine if a port fuel­injection system—including the fuel 
pump, injectors, and fuel­pressure regulator—is operating 
correctly:

1. Attach a fuel­pressure gauge to the Schrader valve on the 
fuel rail. 
2. Turn the ignition key on or 
start the engine to build up 
the fuel­pump pressure (to 
about 35 to 45 psi).
Figure 60–7 Checking the fuel
pressure using a fuel pressure gauge
connected to the Schrader valve.

Continued
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3.

Wait 20 minutes and observe the fuel pressure retained in the fuel 
rail and note the psi reading. The fuel pressure should not drop 
more than 20 psi (140 kPa) in 20 minutes.

If the drop is less than 20 psi in 20 minutes, everything is 
okay; if the drop is greater, there is a possible problem with:
• The check valve in the fuel pump
• Leaking injectors, lines, or fittings
• A defective (leaking) fuel­pressure regulator
To determine which unit is defective, perform the following:
Reenergize the electric fuel pump.
Clamp the fuel supply line, and wait 10 minutes. If the pressure 
drop does not occur, replace the fuel pump. If the pressure drop 
still occurs, continue with the next step.

Continued

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CAUTION: Do not clamp plastic fuel
lines. Connect shutoff valves to the fuel
system to shut off supply and return
lines. 

Figure 60–8 Shutoff valves must be used on vehicles
equipped with plastic fuel lines to isolate the cause of
a pressure drop in the fuel system.

Repeat the pressure buildup of the electric pump and clamp 
the fuel return line. If the pressure drop time is now okay, 
replace the fuel­pressure regulator.
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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TESTING FOR AN INJECTOR PULSE
One of the first checks to perform when diagnosing a no­start 
condition is whether the fuel injectors are being pulsed by the 

computer.
Proper pulsing of the injector is also important in diagnosing a 
weak or dead cylinder.
A noid light is designed to electrically replace the injector in the 
circuit and to flash if the injector circuit is working correctly.
NOTE: The term noid is simply an abbreviation of the word solenoid.

Injectors use a movable iron core and are therefore solenoids. Therefore, a
noid light is a replacement for the solenoid (injector).
Continued
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To use a noid light, disconnect the electrical connector at the fuel 

injector and plug the light into the injector harness connections.
Crank or start the engine. The noid light should flash regularly.
Figure 60–9 (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.

(a)

(b)

Continued
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Possible noid light problems and causes include the following: 

1. The light is off and does not flash  The problem is an
open in either the power side or ground side (or both)
of the injector circuit.
2. The noid light flashes dimly  A dim noid light indicates 
excessive resistance or low voltage available to the injector. 
Both the power and ground side must be checked.
3. The noid light is on and does not flash  If the noid light is 
on, then both a power and a ground are present. Because the 
light does not flash (blink) when the engine is being cranked 
or started, then a short­to­ground fault exists either in the 
computer itself or in the wiring between the injector and the 
computer.
Continued
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CAUTION: A noid lamp must be used
with caution. The computer may show a
good noid light operation and have low
supply voltage.

Figure 60–10 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
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.
NOTE: Some engines require specific procedures to gain access to the
injectors. Always follow the manufacturers’ recommended procedures.

With an ohmmeter, measure the resistance across the injector 
terminals. Be sure to use the low­ohms feature of the digital 
ohmmeter to read in tenths (0.1) of an ohm.
See Figures 60–11 and 60–12. 
Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
By James D. Halderman

Continued
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