Important Preliminary Inspections Before Starting
Perform
Diagnostic System Check - Vehicle
before using the symptom tables, and verify that all of the following are true:
| • | The powertrain control module (PCM) and malfunction indicator lamp (MIL) are operating correctly. |
| • | There are no DTCs stored. |
| • | Verify that 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. |
| • | Verify that the engine coolant temperature (ECT) 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
for diagnosis. |
| • | Verify the customer concern and locate the correct symptom in the table of contents. Inspect the items indicated under that symptom. |
Visual/Physical Inspection
Several of the symptom procedures ask for a careful visual and physical inspection. This step is extremely important. The visual and physical inspection can lead to correcting a condition without further inspections, and can save valuable time. Ensure
that:
| • | The PCM grounds are clean, tight, and in the proper location. |
| • | The vacuum hoses are not split or kinked, and properly connected, as shown on the Vehicle Emission Control Information label. Inspect thoroughly for any type of leak or restriction. |
| • | The mass air flow (MAF) sensor is properly installed. The arrows on the plastic portion of the sensor must point toward the engine. |
| • | The air intake ducts are not collapsed or damaged. |
| • | There are no leaks at the throttle body mounting area, the MAF sensor, or the intake manifold sealing surfaces. |
| • | The ignition wires are not cracked, brittle, or carbon tracked. |
| • | The engine harness wiring and terminals are properly connected and are not pinched or cut. |
Intermittent
Important: Inspect for improper installation of electrical components if an intermittent condition exists. Inspect for aftermarket theft deterrent devices, lights, and cellular phones. Verify that no aftermarket equipment is connected
to the class 2 circuit. If you can not locate an intermittent condition, a cellular phone communication signal may cause the condition.
Important: The condition may or may not turn ON the MIL or store a DTC.
Faulty electrical connections or wiring cause most intermittent conditions. Perform a careful visual and physical inspection of the suspect connectors for the following conditions:
| • | Improperly mated connector halves |
| • | Terminals that are not seated |
| • | Terminals that are damaged or improperly formed |
Reform or replace connector terminals in the affected circuit to ensure proper contact tension. Refer to
Connector Repairs
. Remove the terminal from the connector body in order to inspect for poor terminal wire
connection. Refer to
Testing for Intermittent Conditions and Poor Connections
.
Road test the vehicle with the DMM connected to the suspected circuit. An abnormal reading that is observed when the symptom occurs is a good indication that there is a malfunction in the circuit being monitored.
Use a scan tool to help detect intermittent conditions. Useful features of the GM Techline scan tool include the following:
| • | Trigger the Snapshot feature in order to capture and store engine parameters when the malfunction occurs. Review this stored information in order to see the specific running conditions that caused the malfunction. |
| • | Freeze Frame/Failure Records can also aid in locating an intermittent condition. Review and capture the information in the Freeze Frame/Failure Record associated with the intermittent DTC being diagnosed. Drive the vehicle within the conditions that
were present when the DTC originally set. |
| • | Use the Plot Function on the scan tool to plot selected data parameters. Review this stored information to aid in locating an intermittent condition. Refer to the scan tool Users Guide for more information. |
Important: If the intermittent condition exists as a start and then stall, test for DTCs relating to the vehicle theft deterrent system. Test for improper installation of electrical options such as lights, cellular phones, etc.
Any of the following may cause an intermittent MIL with no stored DTC:
| • | The ignition coils are shorted to a ground or arcing at the ignition wires or the spark plugs. |
| • | The ignition control (IC) wires are 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. |
| • | There is an open diode across the A/C compressor clutch or any other open diodes. |
Use the following tables when diagnosing a symptom complaint:
