2. Clean the throttle body. A dirty throttle body can cause the engine to idle roughly. Remove the throttle body from the engine and clean it with a throttle body cleaner. Be sure to clean the throttle plate and the throttle body bore.
3. Check the idle air control valve (IAC). The IAC valve controls the amount of air that bypasses the throttle plate when the engine is idling. If the IAC valve is dirty or faulty, it can cause the engine to idle roughly. Remove the IAC valve from the engine and clean it with a throttle body cleaner. If the IAC valve is faulty, replace it.
4. Check the spark plugs and spark plug wires. Old or worn spark plugs and spark plug wires can cause the engine to idle roughly. Remove the spark plugs and inspect them for wear. If the spark plugs are worn, replace them. Check the spark plug wires for cracks or breaks. If the spark plug wires are cracked, broken, or loose, replace them.
5. Check the fuel pressure. Low fuel pressure can cause the engine to idle roughly. Connect a fuel pressure gauge to the fuel rail and check the fuel pressure. The fuel pressure should be within the specifications listed in the owner's manual. If the fuel pressure is low, replace the fuel pump or fuel pressure regulator.
6. Have the engine diagnosed by a qualified mechanic. If you have tried all of the above steps and the engine is still idling roughly, take it to a qualified mechanic for further diagnosis.
