Possible Causes & Corrections

Check the following items:

• Low engine coolant level.
• Proper tension on the drive belts.
• Restricted air flow through the radiator or restricted coolant flow.
• Faulty thermostat.
• Check for a correct coolant mixture.
• Inspect for an Engine Coolant Temperature (ECT) sensor that has a shift in value.
• Drive vehicle at the speed of the complaint. Monitoring the fuel trim will help identify the problem.
• If system is lean, the long term fuel trim will be more than 150 on scan tool.
• Check for a poor fuel quality and a proper octane rating.
• If fuel trim readings are normal and there are no engine mechanical concerns, fill fuel tank with a known-good fuel and re-evaluate vehicle performance.
• Inspect spark plugs for proper heat range and gap.
• Check Knock Sensor (KS) system for excessive spark retard activity.
• Inspect ignition timing.
• Inspect for carbon build up. Remove carbon with a top engine cleaner.
• Test combustion chamber pressure by performing a compression test. See CYLINDER COMPRESSION PRESSURE under MECHANICAL INSPECTION under PRELIMINARY INSPECTION & ADJUSTMENTS in BASIC DIAGNOSTIC PROCEDURES - 4.3L ASTRO, BLAZER, JIMMY CANADIAN, SAFARI, SONOMA & S10 PICKUP article.
• Check for proper operation of EGR valve. See DIAGNOSTIC TROUBLE CODE DEFINITIONS in SELF-DIAGNOSTICS - 4.3L ASTRO, BLAZER, JIMMY CANADIAN, SAFARI, SONOMA & S10 PICKUP article.
• Test Torque Converter Clutch (TCC) operation. TCC applying too soon can cause engine to spark knock. See appropriate DIAGNOSIS article in AUTOMATIC TRANSMISSIONS.
• Check Service Bulletins for calibration updates.