GM/Detroit 6.5L ENGINE CONTROLS – Symptoms

Before Using This Section
Before using this section, you shou l d have performed the On-Board Diagnostic System Check and determined that :
1. The control module and the malfunction indicator lamp (MIL) are operating correctly.
2. There are no Diagnostic Trouble Codes (DTCs) stored, or a DTC exists but without a MIL.
Several of the following symptom procedures call for a careful visual and physical check. The visual and physical checks are very important. The checks can lead to correcting a problem without further checks which may save valuable time.

Visual and Physical Checks
Check the following items:
• The control module 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.
• The wiring for the following items:
– Proper connections
– Pinches
– Cuts
• The following symptom tables contain groups of possible causes for each symptom. The order of these procedures is not important. If the scan tool readings do not indicate the problems, then proceed in a logical order, easiest to check or most likely to cause first. In order to determine if a specific vehicle is using a particular system or component, refer to Emission Hose Routing Diagram or Engine Controls Schematics for an application.

Verify the customer complaint.
Locate the correct symptom table. Check the items indicated under that symptom from the following symptom tables:
• Intermittent Conditions
• Hard Start
• Surges/Chuggles
• Lack of Power, Sluggishness, or Sponginess
• Fuel Knock/Combustion Noise
• Hesitation, Sag, Stumble
• Cuts Out, Misses
• Poor Fuel Economy
• Excessive Smoke

Intermittent Conditions
An intermittent condition may or may not turn on the malfunction indicator lamp (MIL) or store a Diagnostic Trouble Code (DTC).

Preliminary Checks
Perform a visual check to locate the cause of the problem.
• Refer to Symptoms.
• The fault must be present to locate the problem. If a fault is intermittent, the use of DTC Tables may result in the replacement of good parts.

Electrical Connections or Wiring
Poor electrical connections or wiring can cause most intermittent problems. Perform a careful check of the suspected circuit for the following:
• Check for poor mating of the connector halves, or terminals not fully seated in the connector body (backed-out).
• Check for improperly formed or damaged terminals. Carefully reform or replace all the connector terminals in the problem circuit to ensure the proper contact tension.
• Check for poor terminal to wire connections. This requires removing the terminal from the connector body to check.

Road Test
If a visual/physical check does not locate the cause of the problem, drive the vehicle with a J 39200 connected to a suspected circuit or use a scan tool. An abnormal voltage or scan reading, when the problem occurs, indicates the problem may be in that circuit.

Intermittent Malfunction Indicator Lamp (MIL)
The following can cause an intermittent MIL and no DTCs:
• Electrical system interference caused by a malfunctioning relay, PCM driven solenoid or switch. They can cause a sharp electrical surge. Normally, the problem will occur when the malfunctioning component is operating.
• The improper installation of electrical devices; such as lights, 2 -way radios, electric motors, etc.
• The Malfunction Indicator Lamp (MIL) circuit intermittently shorted to ground.
• Poor PCM grounds.

Loss of DTC Memory
Check for loss of DTC memory by disconnecting the engine coolant temperature (ECT) sensor and idling the engine until the MIL turns ON. An engine coolant temperature (ECT) DTC should store and remain in memory when turning OFF the ignition for at least 10 seconds. If not, the PCM is malfunctioning.

Additional Checks
Check for an open diode across the A/C compressor clutch and for other open diodes.

R estricted Exhaust System Check
Diagnostic Aids
Proper diagnosis for a restricted exhaust system is essential before any components are rep l aced.
The following procedure may be used for diagnosis:
1, Inspect the entire exhaust system for a collapsed pipe, heat distress, or a possible internal muffler failure.
2. If there are no obvious reasons for the excessive backpressure, the catalytic converter is suspected to be restricted and should be replaced using current recommended procedures. Refer to Engine Exhaust.

A/C Request Circuit Diagnosis
Refer to A/C Controls.
Circuit Description
Turning ON the air conditioning supplies ignition voltage to the A/C compressor clutch and to the PCM to increase and maintain idle speed.
The PCM does not control the A/C compressor clutch. Therefore, if the A/C system does not function, refer to A/C Systems of the appropriate service manual for diagnosis of the system.


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