Lots of reading...
I might suggest that you disconnect the wiring to the idle valve and see what happens. The idle is controlled (is not adjustable) electronically by the valve. In its unpowered, "limp home" setting it should idle at around 900 rpms with no load. It should drop as load is applied (trans, AC, PS, etc). If it were functioning it would start a little more closed at say 750 rpms and as load was added the valve would open to maintain the speed.
With the valve disconnected the only way to get higher rpms is if the valve is stuck open or if there is false air getting into the motor somewhere else (like a throttle sticking or a vacuum leak).
If, with the valve disconnected, the engine still idles at 1500 then remove the valve and stick a cork in one end with a 1/8 inch hole drilled through it and reinstall. If you still have the high idle the problem is not in the valve or its control!
Bosch Master, ASE Master, L1
33 years MB technician