The idle speed spec is 800 RPM plus/minus 100RPM. Prior to carrying out any idle speed adjustment ensure the ignition timing is correctly set. The Nissan adjustment procedure wants you to drive the car until the coolant temperature gauge reads in the middle of the gauge, electric fans have turned on. Switch of the engine disconnect the throttle position switch connector, start the engine and turn the screw (near the bell crank that the accelerator cable attaches too) to achieve an idle speed of 725 to 775 RPM for manual transmission cars and 800 to 850 RPM for automatic transmission cars. Switch of the engine and reconnect the throttle position switch. The computer can then control the idle speed electronically. If the idle speed stays high ensure the coolant thermostat is working correctly, (a defective thermostat will lower fuel MPG, increase engine wear and effect engine idle speed. Adjusting the idle speed will probably only mask some other underlying problem. The idle speed should not really need to be adjusted periodically.