Common issue
Idling problem on most 4AGE engine is most probably due to dirty and clog ISCV (Idle Speed Control Valve). By doing a simple cleaning will solved your idling problem. Dirty Throttle Body may also contribute to bad idling due to clog vacumm hole from TB, based on Mr Chin’s page here.
Basic stuffs
ISC system regulates engine idle speed by adjusting the volume of air that is allowed by-pass the closed throttle valve.
1) Stepper motor
2) Rotary solenoid
3) Duty control ACV
4) On-off control VSV
4AGE 20V uses Rotary solenoid, this is 20V ISCV :

Figure A:ISCV side view, top pipe came from air filter hose and bottom pipe out to Air Intake Chamber. The yellowish plastic on the left, there are 2 screw,unscrew it and adjust the idling.

Figure B: Part number , 22270-16030 for Silvertop and 22270-16070 for Blacktop
Location
ISCV located under the TB, right side…here :

picture taken from www.club4ag.com
or here :

original location : 20v-sensors-locations
or you can refer to my friend page, Mr Chin…here.
How to clean it ?
1) Disconnect the wiring connection and 2 hose , one from intake plenum, at the bottom side and the other one from air intake chamber.
here are the hose from air intake chamber, the biggest hose, in the middle of the picture.

2) Once both hose has been disconnected, put a bottle or any container at the bottom hose (from intake plenum side)…it might be cramp a bit, remove the air filter intake hose would make it easier…no hassle for those using velocity stacks…
once done…spray with carburetor cleaner from the top hose, from air intake chamber…
All the dirty carbon and black particuls came out and fill the container…once satisfied…put everthing in reverse order…remove all tools…start the engine…
wallaa…better idling…
This is what I get after the 1st attempt…wow…!


Then the 2nd attempt…much better…


Good luck and happy DIYing…!
=====================Updated from March 2008======================
Good day…
Under certain circumstances…after cleaning your ISCV the idling still didn’t change much, it may due to other related things…
Which I can think of are :
1) ISCV magnetic part , the yellowish thinggy is totally condemned. Solutions : Replace you ISCV
2) TPS may need to be re-calibrate. Solutions : Will post on how to DIY it, or you may Google for it. Lots of page or Forum have discussed it.
or
3) ISCV wasn’t properly clean, still got a few residue inside. Solutions : Jeng jeng jenggggg !!! Please read more…Oyeh oyeh…
Ok…now there will be the time where above mentioned method won’t work, as the residue gets too thick to be clean that way…
So you have to totally take it out.
How ?
Kindly read on this : Twincam.org
Credit to TERRA Operative.
Now since you’ve read it all…
Don’t forget to do the marking on this part :
(between the “metal slider” n ISCV body)

Now…Here’s the bits and pieces of what ISCV, up close…

Now on the main ISCV body, there’s a small raotray “nipple” on the right side of it.
Try to turn it, you might feel some hesitation.
It due to this culprit :

(Close position)

(Open position)
After you’ve cleaned it with a Carb Cleaner…you can feel, there’s no more hesitations. Free play…woo hooo ! Done !
Here the carbon-clogging-dirt-make-our-idling-sucks stuffs :

(ignore the water droplets)

Put it back all together in reverse steps, make sure everthing connected as it is. Then start the engine, see how’s the results.
Happy DIYing… !