Idles at 2500 rpm

spray carb cleaner w/eng running at idle on all vac lines if eng revs up u got vac leak. may have intake crack. sticky cables ? be careful w/spray it is very flammable.

Check IAC and clean.

wow a 99 an oldie but a goodie , one that old id be looking at all the vac lines , for pin holes,worn spots and cracks , a vac leak will make engne run rough at idle and as you acclerate the vac goes away so hole in hose does not matter

which car is it?

problem with 1993 Geo Prizm '99 silverado 5.3 engin will not idle runs at 2500 rpm and drops down to 1000 rpm the back up to 2500 rpm -------------------------

The idle speed is not supposed to be adjustable. It is controlled by the computer.
What you may have is a vac leak, or the throttle cable may not be adjusted correctly.
If the throttle plate is completely closed, and there are no vac leaks, the idle air control valve is supposed to control idle speed by regulating how much air enters the intake.

could be a vac leak or injector problem

this sometimes occurs with age due to egr and intake becoming carbon filled , inturn requireing more vacuum inturn raising rpm to keep up , check all vac lines for poss leaks this will cause prob , to check for leaks crank engine let idle use can of wd40 or starting fluid and spray lightly around suspected area , if rpms raise when u spray then u have found ur leak

check for vacuum leaks and air by-pass valve sticking......

i would first check the timing and if that does not fit i would take the cover back off and look at the belt for a bad or missing tooth

there is a idle air control motor that is dirty you can try to clean it with throttle body cleaner or replace it check to make sure you have no vac leaks first