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 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 customers 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
condition without further tests and can save valuable time. This check
includes:
| • | 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. Refer to
Emission Hose Routing Diagram
. |
| • | 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 engine harness wiring and terminals for proper connections,
pinches or cuts. Refer to
Harness Routing Views
in Wiring Systems. |
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 condition 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 conditions. The fault must be present in order to locate
the condition.
Faulty electrical connections or wiring cause most intermittent conditions.
Perform a careful visual/physical check for the following conditions:
| • | Reform or replace connector terminals in the circuit in order
to insure proper contact tension. Refer to
Repairing Connector Terminals
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 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 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 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 stall, check
for DTC(s) 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 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 concern.
