Idles too high
 

I've got a 2001 Ranger with the 2.3 4cyl.that wont idle right. It jumps to 2000 rpm when you start it, stays there and then as it warms up it slowly drops to the right speed. It does this when I'm driving also. Turning the AC on helps a little but it just won't run right. I've changed the mass air flow sensor and the throttle position sensor but it still acts up. What am I missing?

Hello! I also just recently had this issue! When i cleaned my throttle body and idle air control valve it helped it out a lot. Mine is not completely fixed because my idle air control does not seem to be working so i am about to change that and i believe it should be good to go. Mine was also throwing a code (p1506) which indicated that the IAC is sticking or not working properly.

Actually the P1506 Idle Air Control Overspeed Error
Means that the voltage that the computer is sending to the IAC Valve is not lowering the RPMs like it should

IAC Valve is a solenoid, it has a metal rod with a valve at the end, and then a coil of wire that can be changed into an electro-magnet by running voltage thru the coil
The metal rod has a spring that holds it out, away from the coil, holds valve closed in this application
As voltage is applied to the coil magnetic force pulls the metal rod back against the spring pressure, by varying the voltage the metal rod can be held or moved into position, in this case to let in more or less air to set a stable idle

There is no "preset" voltage, i.e. 7volts to IAC = 800rpms, the computer "learns" the correct voltage to send to IAC Valve to get a specific RPM and to hold it
The computer has preset IDLE RPMs in memory, for cold and warm engine temp, called "target idles" set by emissions standards usually, lol

IAC Valve gets full 12volts with key on, red wire, the Ground wire runs to the computer, the computer uses PWM(pulse width modulation) to control voltage by pulsing the Ground
This is used because vehicles have varying voltages, battery is 12.3 to 12.8volts, engine running and alternator has 13.5 to 14.8volts
Using PWM modulation on ground wire allows precise control regardless on input voltage

P1506 can mean computer has lowered the voltage to IAC Valve but engine is staying at High RPMs
So could be a vacuum leak
Or as you said a stuck valve

I just had this issue sort of. It would start and idle fine but rev it a bit and all of the sudden the rpms would stay high at idle and or when ever holding the clutch in. I cleaned. The IAC multiple times. Took off and cleaned throttle body a long with iair intake but honestly what seems to have solved it for me .. as stupid as this might be but my EGR sensor. It had a graphite gasket so when I removed it prior to having the issue I didn't actually scrape the gasket off and instead I just bolted it right back on thinking it was good turns out the graphite that was left on the EGR valve was preventing a proper seal and after I removed it once more and ended up scraping the old graphite gasket off I bolted it back on and I had it on loosely at first and started the engine and the idle stayed high but as I slowly started bolting it completely back onto the intake manifold I noticed the RPMs started dropping until I got it fully on and the RPMs had returned to normal with no issue high idle