Important Preliminary Checks Before Starting
Perform the
Powertrain On Board Diagnostic (OBD) System Check
before using the Symptom Tables, and verify that all of the
following conditions are true:
| • | The PCM and the Malfunction Indicator Lamp (MIL) are operating
correctly. |
| • | There are no DTCs stored. |
| • | 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 the engine temperatures from reaching damaging levels.
The system perceives Engine Coolant Over temperature as a lack of power,
a miss, or a 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, and 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 their proper location |
| • | The 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 the throttle body mounting area, the Mass Air Flow
(MAF) sensor, and the intake manifold sealing surfaces |
| • | The ignition wires for cracking, for hardness, for proper routing
and for 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, for lights
and for cellular phones. Ensure that no aftermarket equipment is connected
to the Class 2 circuit. If you cannot 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.
Poor electrical connections or improper wiring cause most intermittent
problems. Perform a thorough visual/physical check for the following conditions:
| • | An improperly formed or damaged terminal, refer to
Connector Repairs
in Wiring Systems. |
| • | Reform or replace the connector terminals in the problem circuit
in order to insure the proper contact tension. Refer to
Connector Repairs
in Wiring Systems. |
| • | Poor terminal to wire connection requires removing the terminal
from the connector body in order to check. |
Road test the vehicle with a 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 in
order 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. |
| • | A Scan Tool's Freeze Frame buffer or Failure Records buffer can
also aid in locating an intermittent condition. Review and capture the information
in the Freeze Frame and/or Failure record that is associated with the intermittent
DTC being diagnosed. Drive the vehicle in the conditions that were present
when the DTC originally set. |
Important: If the intermittent condition exists as a start and then a stall, check
for any DTCs relating to the 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 ground, arcing at the ignition wires
or the spark plugs |
| • | The PCM grounds, refer to PCM wiring diagrams. |
| • | The Ignition Control (IC) wires routed too 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.
