Symptoms
Symptoms
Important Preliminary Checks Before Starting
Perform the Powertrain On Board Diagnostic (OBD) System Check 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 DTCs stored.
* Ensure that the engine coolant temperature is not above 130°C (266°F). This condition causes the PCM to operate in engine shut down feature.
All medium duty trucks that are equipped with the gasoline engines, have one of the following engine shut down features.
The standard shut down feature (RPO UER), or the optional engine shut down feature (RPO KPJ).
Refer to Engine Shutdown Feature Engine Shutdown Feature.
* The scan tool data is within the normal operating range, refer to Scan Tool Data List Scan Tool Data List.
* Verify the customer complaint, and locate the correct symptom in the table of contents. Inspect the items indicated under that symptom.
Visual/Physical Check
Important: Several of the symptom procedures ask for a careful visual and physical check. This step is extremely important, it can lead to correcting a problem without further checks and can save valuable time.
* Inspect the PCM grounds for being clean, tight, and in their proper location.
* Inspect the vacuum hoses for splits, kinks, and proper connections, as shown on the Vehicle Emission Control Information label. Inspect thoroughly for any type of leak or restriction.
* Inspect the mass air flow (MAF) sensor installation. The arrows on the plastic portion of the sensor must point toward the engine.
* Inspect the air intake ducts for being collapsed or for having damaged areas.
* Inspect for air leaks at throttle body mounting area, MAF sensor, and the intake manifold sealing surfaces.
* Inspect the ignition wires for cracking, hardness, proper routing and carbon tracking.
* Inspect the engine harness wiring and terminals for proper connections, pinches or cuts.
Intermittent
Important: Inspect 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.
* Inspect for an intermittent 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 and 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 DMM 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 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. Inspect for improper installation of electrical options such as lights, cellular phones, etc.
* Inspect installation of components.
* 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 PCM grounds, refer to PCM wiring diagrams.
- 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:
- Hard Start Hard Start
- Surges/Chuggles Surges/Chuggles
- Lack of Power, Sluggishness, or Sponginess Lack of Power, Sluggishness, or Sponginess
- Detonation/Spark Knock Detonation/Spark Knock
- Hesitation, Sag, Stumble Hesitation, Sag, Stumble
- Cuts Out, Misses Cuts Out, Misses
- Poor Fuel Economy Poor Fuel Economy
- Rough, Unstable, or Incorrect Idle and Stalling Rough, Unstable, or Incorrect Idle and Stalling
- Dieseling, Run-On Dieseling, Run-On
- Backfire Backfire