Important Preliminary Checks Before Starting
Refer to
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. |
| • | Verify the customer complaint, 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 their 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, proper routing 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). |
| • | Improperly formed or damaged terminal. |
| • | Reform or replace connector terminals in the problem circuit in
order to insure proper contact tension. |
| • | 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 a defective relay, PCM driven solenoid, or a switch that can cause an
electrical system interference. Normally, the problem will occur when the
faulty component is operating. 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 for being shorted to 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.
