High erratic idle issues, Idling between 1400-2000 RPM Options

[blu94gt]

So here's the deal with the idle, I searched and couldn't find too much helpful info:
Been having some high idle problems for a couple months. Car will idle very high, not go down when the engine warms up, etc. Put up with it for a few weeks since I don't do much stop-and-go driving. Get stuck in traffic one day in the beginning of January and come to figure out that I can rev the engine up with the brake pedal, close to 2500 RPM just from pumping the brakes. So I figure I have a leaky brake booster letting more air into the engine, so I pull the line to it and cap the inlet on the intake manifold. Idle immediately drops from a pretty consistent 1800 or so to 1100-1200. Fine, that's better at least.

So after that I cleaned my TB again, sprayed some carb cleaner in the idle control, and no change. Car will idle high, and every now and then after I turn it off and on again it will idle around 800-900 RPM. Lately it's been idling between 1400-1800 most of the time, and sometimes will dip down. My gas mileage has been around 23-24 mpg, can't find a vacuum leak, and the car is running really strong otherwise. New plugs, wires, distributor cap and rotor just last week. Oh and the car is a 99 GT, with a stock 5sfe, almost 180k miles.

Just want to get some input before I order an IAC unit for a few hundred bucks.

Cliffs:
idles between 1400-1800 even when completely warm, driving for over 2 hours
brake booster disconnected, was causing idle to be even higher
every now and then will idle closer to normal around 800-900
can't find a vacuum leak
replaced plugs, wires, distributor cap and rotor last week
think it's IAC?

[4-eyed-freek]

iac for sure same thing happened to me, also i had a bit of water in the fuel tank, put some HEAT in and all was good, also idk if you have the coolant bypass done to your car, if so i would go back to stock because that will help alot with your idle problem! you have all the symptoms i had, one day it felt like it would mabey die, and then idle fine, and then idle really high. but it would still run fine down the road. i would also change you fuel filter.........things i did when this happend to me, took out the iac, cleaned it. heat through the gas tank, and changed the fuel filter. after all this everything was all good. i had also changed the wires plugs and oil to see if it would help.

This post has been edited by 4-eyed-freek: Feb 9, 2011 - 10:42 PM