Important Preliminary Checks Before Starting
Perform the
Powertrain On Board Diagnostic (OBD) System Check
before using the Symptom Tables, and verify all of the following
are true:
| • | The PCM and MIL (Service Engine Soon) are operating correctly. |
| • | There are no DTC(s) stored. |
| • | Ensure the engine is not in a torque reduction mode. The PCM turns
certain injectors off and/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/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, Mass Air Flow (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/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 Digital Multimeter (DMM) J 39200
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 Tools Freeze Frame or Failure Records can also aid
in locating an intermittent condition. Review and capture the information
in the Freeze Frame and/or 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 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 DTC(s) 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.
