Chapter
E
: Computerized Engine Control Diagnosis And Repair (Including OBD II)
87
The result?
The customer returns the next day with the same
symptoms. Only then does the technician examine
further to determine that a faulty injector is leaking
when the engine is turned off. This leak floods a
cylinder. The flooded cylinder causes its associated
spark plug to fail. This would also drain the fuel rail,
causing extended cranking on a cold start.
This example clearly illustrates that a systems-based
approach to the diagnostic process is vital to help
eliminate multiple and repeated component failures
that result in dissatisfied customers.
When approaching any diagnostic problem, take the
time to define vehicle symptoms. How is the vehicle
running? Does it have rich symptoms like poor gas
mileage or a failed emissions test? Does it surge or idle
rough? Is it hard to start? Next, do a thorough
inspection for obvious problems such as vacuum leaks
and damaged electrical connections. Don't forget to
consider the basics such as low fuel pressure,
incorrect ignition timing, low or uneven engine
compression, and fuel quality. If possible, a review of
recent vehicle service may yield valuable diagnostic
clues. For recent vehicle service information, check
dealership resources and communicate with the
vehicle owner.
Connect the scan tool and retrieve stored DTCs and
freeze frame data. Record your findings, then check
the service manual to learn the specific conditions that
cause the DTC. Take the time to thoroughly understand
what caused the DTC.
Check the readiness status of system monitors. If the
readiness status is "NO" for all monitors, review recent
service history; the battery may have been changed or
the vehicle may have been in another shop where
DTCs were erased. The vehicle must be driven through
the complete drive cycle to ensure all monitors run. If
readiness status is "NO" for only one or two sensors,
check sensors, actuators, and related circuitry for
problems that would prevent the monitor from running.
Again, the vehicle may have to complete an entire drive
cycle to provide the time and conditions to run the
remaining monitors.
When there is more than one DTC in memory, diagnose
and correct component-related DTCs before
diagnosing system failure DTCs. A sensor or actuator
problem may prevent a monitor from running or cause
a system to fail the monitor. Once a component failure
is repaired, drive the vehicle through the specified drive
cycle to be sure the system is fully repaired. For
example, when discovering a code P0125, "excessive
time to enter closed loop" and a code P0155,
"HO2S/1, Bank 2 Heater Malfunction", the best
procedure is to diagnose and repair the HO2S/1 heater
malfunction first, even though its DTC is a higher
number.
Next, clear codes and drive the vehicle as directed in
the service manual. In this example, it is probable that
the failed oxygen sensor heater caused the system to
be slow entering closed loop. Misfire and fuel control
DTCs are considered priority codes and should always
be diagnosed first.
When using the drive cycle to confirm repairs, review
freeze frame data for the driving conditions present at
the time the DTC was recorded. It is especially
important when confirming misfire and fuel control
repairs to closely match the engine rpm, calculated
load, and engine temperature values recorded in the
freeze frame.
How close is close? Before the PCM will deactivate the
MIL for misfire and fuel control codes, engine speed
must be within 375 rpm of the engine speed when the
code was set, and the calculated load value must be
within ±10% of the load present when the code was
set. Be aware that some manufacturers may direct you
to drive a portion of the drive cycle to confirm a
particular repair. Drive cycles vary between
manufacturers and must always be followed exactly.
Freeze frame and scan tool data must be analyzed with
care. Use service manuals to learn the normal
parameters for each sensor and actuator. Review
freeze frame data to identify all sensors and actuators
that are out of range. Many times a sensor will be out
of range and not set a DTC, especially when the out-
of-range sensor is responding to an unusual condition.
Try to determine if the suspect sensor is reporting an
unusual vehicle condition or sending a signal that
doesn't match the actual symptoms or other sensor
data. When you have gathered all necessary
information: vehicle symptoms, driving conditions,
DTCs and sensor/actuator data, use your knowledge
and experience to pick out the most probable cause of
the symptom. Always refer to appropriate service
manuals for proper test procedures when testing
sensors, actuators and related circuits.
Once all of the preliminary checks have been made,
check for codes stored in memory. To access PCM
memory, the system must be placed in diagnostic
mode. Procedures for entering diagnostics varies by
manufacturer and system. Most systems display codes
as a series of flashes on the MIL, or on the display
screen of a scan tool. The following paragraphs outline
the common features and principles of trouble code di-
agnosis.
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