Possible Causes & Corrections

Check the following items:

• Inspect the Heated Oxygen Sensor (HO2S). Check for silicone contamination from fuel or improperly used sealant. The sensor will have a White powdery coating. The sensor will result in a high but false signal voltage or rich exhaust indication. The control module will reduce the amount of fuel delivered to the engine causing a severe driveability condition.
• Inspect Engine Coolant Temperature (ECT) sensor using scan tool in order to compare the engine coolant temperature with the ambient air temperature on a cold engine. If coolant temperature reading is more than 5 degrees more than or less than the ambient air temperature on a cold engine, test for a high resistance in the coolant sensor circuit or the sensor itself. Inspect the Manifold Absolute Pressure (MAP) sensor response and accuracy.
• Check fuel pressure. See FUEL SYSTEMS in BASIC DIAGNOSTIC PROCEDURES - 4.3L SIERRA & SILVERADO article.
• Determine if a rich or lean system causes the condition. Drive vehicle at the speed of the complaint. Monitoring the fuel trim will help identify the condition. Using scan tool, observe fuel trim conditions. A lean condition will indicate a long term fuel trim of more than 150 on scan tool. A rich condition will indicate a long term fuel trim of less than 115 on scan tool.
• Perform fuel injector balance test. See FUEL SYSTEMS in SYSTEM & COMPONENT TESTING - 4.3L SIERRA & SILVERADO article.
• Test fuel injector driver circuit. Disconnect injector harness connector. Connect an injector test light between the terminals of each injector connector and observe test light while cranking. If test light fails to blink at any connector, injector drive circuit harness connector or terminal is faulty.
• Perform fuel injector coil test. See FUEL SYSTEMS in SYSTEM & COMPONENT TESTING - 4.3L SIERRA & SILVERADO article.
• Inspect Evaporative Emission (EVAP) control system. See DIAGNOSTIC TROUBLE CODE DEFINITIONS in SELF-DIAGNOSTICS - 4.3L SIERRA & SILVERADO article.
• Check ignition output using Spark Tester (J-26792).
• Inspect spark plugs. Check for wet plugs, cracks, wear, improper gap, burned electrodes, blistered insulators or heavy deposits.
• Check spark plug wires. Resistance should be 30,000 ohms per foot.
• Inspect for vacuum leaks. Vacuum leaks can cause a higher than normal idle and low Idle Air Control (IAC) counts.
• Inspect control module grounds for being clean, tight and in their proper locations.
• Observe scan tool to determine if the control module is receiving an A/C signal. If a condition exists with the A/C ON, inspect the A/C system operation.
• Check EGR being ON while idling which will cause roughness, stalling and hard starting.
• Inspect battery cables and ground straps for being clean and secure.
• Inspect the crankcase ventilation valve for proper operation by placing a finger over the inlet hole in the valve end several times. The valve should snap back. If not, replace PCV.
• Broken motor mounts.
• Improper valve timing.
• Low compression. See CYLINDER COMPRESSION PRESSURE under MECHANICAL INSPECTION under PRELIMINARY INSPECTION & ADJUSTMENTS in BASIC DIAGNOSTIC PROCEDURES - 4.3L SIERRA & SILVERADO article.
• Worn rocker arms.
• Broken or weak valve springs or worn camshaft lobes.