When I first bought my 'Stang I always had problems with the idle. No matter what I did it always went up and down, up and down, up....uh, you get the point. After performing a full tune up, which included changing the spark plugs, plug wires, fuel filter, air filter (k&n), cap, rotor, and setting the timing and tps voltage I was disappointed to find that the car still had a lousy idle. So what's next? Try changing the idle air bypass solenoid (IAB)? I was about to, until I noticed that the throttle plate in the Throttle body was so dirty that you could barely see the edges of the plate. I guess the guy who owned it before me never cleaned it. I guess I found the problem..... So I went to the parts store and got a can of carb/throttle body cleaner and got to work. After I was done the stang had a better idle than most Honda's I've driven.
Here's how it's done
Before you do anything you will want to start the car up and let it get to operating temperature so the dirt in the throttle body will come loose.
The first thing to do is to take the rubber intake hose off by loosening the clamp that holds it to the throttle body. All you need, to do this is a regular flat-head screw driver.
Next you have to pull the hose off, and push it out of the way so you have some room. If you want to make things easier on yourself, then you might want to take the hose totally off by loosening the clamp that holds the hose to the mass air sensor. Also make sure you place a rag or paper towel under the throttle body so the cleaner doesn't go everywhere.
Then take the throttle body/carb cleaner and spray away. Hold the throttle plate open and spray it all around inside the throttle body.
After you are done put everything back together and check to make sure you didn't disconnect any vacuum lines by mistake when you pushed the intake hose out of the way. I did ;)
Now for the IAB motor. First you have to disconnect the wires going to the IAB. Then remove the two 8mm screws that hold the motor to the throttle body. When you take it off make sure you don't lose the gasket that goes between the two.
On the left is a picture of what the IAB motor looks like when its off the throttle body. To clean it just spray the cleaner in those two holes (arrows). You can let it soak for a while then spray more in it. While you're at it spray some cleaner in the the two holes on the throttle body too.
That's all there is to it, now put everything back together and start her up. It probably won't start right up as it usually does, so don't get nervous. Finally let the car run for a few minutes and your done.