I need some help from you mechanics...

I keep getting a P0507 mil code, which is "idle is too high" even though my car is purring at 650 rpm with the AC off. However, when I turn the AC/Heat on, the rpms shoot up to 950-1000 rpm. I believe the ICV (idle control valve) is that part that sits on the back of the plenum and receives two hoses. One hose from the neck of the plenum and the other hose comes from the evap. It hose from the evap connects to a valve that reads "EVAP SERVICE PORT 1 PSI MAX". I know the ICV is functioning, because it's keeping the idle at 650 rpm when the AC/Heat is off. I disconnected the evap hose while the car was running and the AC/heat was on and there was absolutely no air coming in or out of the hose. I tried the same thing with the hose from the plenum and it was sucking in air. This leads me to the conclusion that the valve needs air from the evap hose to let the ECU know that the AC/Heat is on. Without that air, the car idles high and the ECU throws a P0507 code.

What I need to know is what would cause no air to travel through the evap hose? Also, is this an easy fix? My Infiniti service department thinks my Crawford plenum has a vacuum leak, but I checked and it is not leaking. I can't blame them. A high idle could be initially blamed on a vacuum leak, but it should be high with the AC/Heat on or off.