hootenanny 1/31/2013On the bottom of the throttle body there is a round thermo-wax element that when cold extends a plunger and opens the throttle slightly so that cold engine automatically idles faster. As the engine coolant warms up and begins melting the wax inside the thermo-wax element, the plunger begins to retract and move away from the throttle allowing the engine idle speed to slowly return to a base/warm idle speed of approx. 700 to 800 RPM. The Thermo Element (kit) at Nissan dealership is approx. $150.00 for the part.
After the engine is fully warmed up,..various increased engine load situations caused by increased electrical load on alternator or mechanical load such as turning on A/C, turning steering wheel (p/s pump load) etc.,..the idle speed is then computer controlled by the IACV (Idle Air Control Valve) which is buried underneath the rear of the intake manifold. Sometimes removing it and cleaning it's spring load pintle and passages will restore it's operation,..but since the removal of the entire upper intake manifold plenum is required to access the IACV,..it's probably best to just install a new one (approx. $200.00 dealership) to eliminate the chance it may fail and have to remove the manifold plenum again.