For 1990-2009 cars

### Important Preliminary Checks Before Starting

Perform the Engine Controls Diagnostic Check before using the symptom tables. Refer to Diagnostic System Check - Engine Controls . Verify all of the following are true:

• | The powertrain control module (PCM) and malfunction indicator lamp (MIL) Service Engine Soon are operating correctly.

• | There are no diagnostic trouble codes (DTCs) stored.

• | The scan tool data is within the normal operating range. Refer to Scan Tool Data List .

• | Verify the customer 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 and 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 the 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, MAF sensor, and the intake manifold sealing surfaces

• | The ignition wires for cracking, hardness, 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 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

• | An improperly formed or damaged terminal

• | Reform or replace connector terminals in the problem circuit in order to insure proper contact tension. Refer to Connector Repairs in Wiring Systems.

• | Remove the terminal from the connector body in order to check for poor terminal to wire connection . Refer to Testing for Intermittent Conditions and Poor Connections in Wiring Systems.

Road test the vehicle with the 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 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 tool 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 in the conditions that were present when the DTC originally set.

• | You can use the Plot Function on the scan tool in order to plot selected data parameters. You can then review this stored information. This may aid in locating an intermittent problem. Refer to the scan tool Users Guide for more information.

Important: If the intermittent condition exists as a start and then stall, check for DTCs relating to the vehicle 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 PCM grounds--Refer to Engine Controls Schematics .

• | 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

• | Surges/Chuggles

• | Lack of Power, Sluggishness, or Sponginess

• | Detonation/Spark Knock

• | Hesitation, Sag, Stumble

• | Cuts Out, Misses

• | Poor Fuel Economy

• | Rough, Unstable, or Incorrect Idle and Stalling

• | Dieseling, Run-On

• | Backfire