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

[azian_advanced]

everything you say seems like it's an iac valve issue except how it randomly idles back down to 800-900 every now and then.
i'd start by taking off the iac valve anyway and clean it if its clogged with gunk or if the valve has trouble turning. i just did mine not too long ago and it wasn't too difficult. it solved my low idle issue on cold start up because the valve was stuck shut so i'm guessing yours might be stuck open??

[DGolden32]

I had the same thing, and if I punched the gas really quick and let off it would go back down til I revved back up slowly again. It was the iac