Also, the idle air control valve may be sticking but you need a scanner for testing, not a code reader. Question,, is the transmission shifting like it should? IF not, possible throttle position sensor out of range which will also cause high idle. To test it a scanner is needed again, so i suggest have it scanned to check live sensor data for your problem.
The first thing that comes to mind is the intake gasket failures on these engines. They used victor gaskets made with plastic. The plastic with rubber combination fails in time. Try some water in a spray bottle, and spray a heavy dose of water around the intake sealing area, at the valve cover seal. Try around the throttle body gasket area, and around the vacuum hoses. With engine running the idle should change if water is introduced at the leaking area. Let me know.