Sensor/System Checks

Check the Knock System (KS) for excessive spark retard. See KNOCK SENSOR under SENSORS & SWITCHES in SYSTEM & COMPONENT TESTING - 4.0L AURORA article.
Check the Exhaust Gas Recirculation (EGR) system for proper operation. See EXHAUST GAS RECIRCULATION under EMISSION SYSTEMS & SUB-SYSTEMS in SYSTEM & COMPONENT TESTS - 4.0L AURORA article.
Check for a dirty or damaged air cleaner element.
Check for collapsed, or damaged intake air ducting.