Possible Cause & Correction

Check the following items:

• Ensure throttle linkage and/or TP sensor is not sticking or binding. Ensure throttle bore is free of foreign material.
• Check for vacuum leaks.
• Check engine idle speed (both base idle and PCM idle).
• Check Idle Air Control (IAC) system. Check for vacuum leak or high idle. Check for alcohol contaminated fuel, incorrect fuel pressure or a restricted injector. Check for foreign material in IAC bore of throttle body. Check for proper connection of IAC valve electrical connector. Check for failure of A/C compressor or relay.
• Check for proper operation of EGR system.
• Check Transaxle Range (TR) switch circuit. Ensure TR switch is properly adjusted.
• Check Power Steering Pressure (PSP) switch operation.
• Check charging system output. Repair charging system if voltage is less than 9 volts or more than 16 volts.
• Check for fuel in pressure regulator vacuum hose. If fuel is present, replace regulator.
• Check evaporative emission control system.
• Check for proper spark plug gap.
• Check PCM grounds for clean and tight connections.
• Check A/C signal to PCM. If problem exists only when A/C is on, check A/C system operation and pressures.
• Ensure the installed PCM program is correct for the particular vehicle. Check with dealer for latest application information.
• Check Manifold Absolute Pressure (MAP) or Mass Airflow (MAF) sensors for proper operation.
• Check Heated Oxygen Sensor (HO2S) operation. Check for silicone contamination or incorrect RTV sealant.
• Check for excessive fuel additives.
• Check for shorted or open injector windings.
• Check for leaking injectors. Perform injector balance test. See FUEL INJECTOR BALANCE TEST under FUEL SYSTEMS in appropriate SYSTEM & COMPONENT TESTING article.
• Check injector harness locations. Ensure injectors are connected to correct injector/cylinder according to firing order.
• If rough idle only occurs when engine is hot, check PCV valve operation. Check for manifold vacuum at inlet end of crankcase vent tube assembly with engine idling.
• Using a scan tool, monitor the Knock Sensor (KS) system for any excessive spark retard activity.
• Test the crankcase ventilation valve for proper operation. Place a finger over the inlet hole of the valve end several times. The valve should snap back. If not, replace the valve.
• Check ignition system. Check for moisture, dust, cracks, burns, etc. Check for shorts and faulty insulation by spraying spark plug wires with a fine water mist.
• Using Spark Tester (J-26792), check for available secondary voltage.
• Check to see if condition is caused by engine running either rich or lean.
• Check engine compression. Check for incorrect valve timing, bent push rods, broken valve springs or worn camshaft lobes.
• Check Engine Coolant Temperature (ECT) sensor for proper temperature-to-resistance values.
• Check exhaust system for restrictions, such as a damaged or collapsed pipe, muffler or catalytic converter.
• Check the Torque Converter Clutch (TCC) operation. A scan tool should indicate an RPM drop when the TCC is commanded ON.
• Check the charging system. A bad diode in the alternator can allow A/C noise to enter the electrical system. This may affect the engine controls system operation.
• Check the transaxle range switch circuit. Ensure the scan tool indicates the vehicle is in drive with the gear selector in Drive or in Overdrive.
• Check for weak or broken motor mounts.