Important Preliminary Checks Before Starting
Perform
Powertrain On Board Diagnostic (OBD) System Check
before using the symptom tables, and verify
all of the following are true:
| • | The powertrain control module (PCM) and malfunction indicator
lamp (MIL) Service Engine Soon are operating correctly. |
| • | There are no DTCs stored. |
| • | Ensure the engine is not in a torque reduction mode. The PCM turns
certain injectors OFF or reduces the engine timing when the PCM detects
an over torque condition or an abusive maneuver. Refer to
Powertrain Control Module Description
for further information. |
| • | Ensure that the engine coolant temperature is not above 130°C
(266°F). This condition causes the PCM to operate in Engine Coolant
Over Temperature-Fuel Disabled Mode. While in Engine Coolant
Over Temperature-Fuel Disabled Mode, the PCM turns the fuel
OFF to four cylinders at a time in order to keep engine temperatures
from reaching damaging levels. The system perceives Engine
Coolant Over Temperature as a lack of power, miss, or rough
idle. If the vehicle is operating in Engine Coolant Over Temperature-Fuel
Disabled Mode, refer to
Engine Overheating
in Engine Cooling for diagnosis. |
| • | Verify the customer concern and locate the correct symptom in
the table of contents. Check the items indicated under that symptom. |
Visual/Physical Check
Several of the symptom procedures ask for a careful visual and physical
Check. This step is extremely important. It can lead to correcting a
problem without further checks and can save valuable time. This
check includes:
| • | The PCM grounds for being clean, tight, and in the proper location |
| • | Vacuum hoses for splits, kinks, and proper connections, as shown
on the Vehicle Emission Control Information label -- Check thoroughly
for any type of leak or restriction. |
| • | The mass air flow (MAF) sensor installation -- The arrows
on the plastic portion of the sensor must point toward the engine. |
| • | The air intake ducts for being collapsed or for having damaged
areas |
| • | Air leaks at throttle body mounting area, MAF sensor, and the
intake manifold sealing surfaces |
| • | The ignition wires for cracking, hardness, and carbon tracking |
| • | The engine harness wiring and terminals for proper connections,
pinches or cuts |
Intermittent
Important: Check for improper installation of electrical components if an intermittent
condition exists. Inspect for aftermarket theft deterrent devices, lights,
and cellular phones. Ensure that no aftermarket equipment
is connected to the Class 2 circuit. If you can not locate
an intermittent condition, a cellular phone signal communication
may cause the condition.
Important: The problem may or may not turn ON the malfunction indicator lamp (MIL)
or store a DTC. DO NOT use the diagnostic trouble code (DTC) tables for
intermittent problems. The fault must be present in order
to locate the problem.
Faulty electrical connections or wiring cause most intermittent problems.
Perform a careful visual and physical check for the following conditions:
| • | Poor mating of the connector halves or a terminal not fully seated
in the connector body backed out |
| • | An improperly formed or damaged terminal |
| • | Reform or replace connector terminals in the problem circuit in
order to insure proper contact tension. Refer to
Connector Repairs
in Wiring Systems. |
Road test the vehicle with the (DMM) connected to the suspected circuit.
An abnormal voltage when the malfunction occurs is a good indication that
there is a malfunction in the circuit being monitored.
Use a scan tool in order to help detect intermittent conditions. The
GM Techline equipment scan tool has several features that you can use
to locate an intermittent condition. Use the following features
to find intermittent faults:
| • | You can trigger the Snapshot feature in order to capture and store
engine parameters within the scan tool when the malfunction occurs. You
can then review this stored information in order to see what
caused the malfunction. |
| • | Using a scan tool Freeze Frameand Failure Records can also aid
in locating an intermittent condition. Review and capture the information
in the Freeze Frame and Failure Record associated with the
intermittent DTC being diagnosed. Drive the vehicle in the conditions
that were present when the DTC originally set. |
| • | You can use the Plot Function on the scan tool in order to plot
selected data parameters. You can then review this stored information. This
may aid in locating an intermittent problem. Refer to the scan
tool Users Guide for more information. |
Important: If the intermittent condition exists as a start and then stall, check
for DTCs relating to the vehicle theft deterrent system. Check for improper
installation of electrical options such as lights, cellular
phones etc.
Any of the following may cause an intermittent malfunction indicator
lamp (MIL) with no stored DTC:
| • | The ignition coils shorted to a ground, arcing at the ignition
wires or the spark plugs |
| • | The ignition control (IC) wires for being routed to close to the
secondary ignition wires, coils, or the generator -- Ensure that all
of the circuits from the PCM to the ignition coils have good
connections. |
| • | An open diode across the A/C compressor clutch and other open
diodes |
Use the following tables when diagnosing a symptom complaint:
