For one, the FITV(fast idle thermal valve) comes into play when the engine is cold. it will hold the idle until the coolant reached a specific temperature and the plunger inside will start to close. once warmed up, the IACV(idle air control valve) takes over from there. I just re-assembled my own fitv due to idle surge.

There is an easy way to diagnose this, and than there is a not so easy to diagnose this.

1) remove your intake piping from the throttle body.
2) look inside the throttle body. you will see 2 holes inside the throttle body. The top hole is for your IACV. The bottom hole is for your FITV.
3) with the engine running, place your finger over the IACV hole.
If your idle corrects itself than your IACV is your problem.

4) put your finger over the FITV hole. Again, if the idle corrects itself, than the FITV is at fault. It most likely isnt broken, but probably just unthreaded itself due to vibration.

IF its the FITV, it sits just below the throttle body and has only 2 coolant lines. One in, and one out. When you remove the coolant lines, be sure to have some vise grips ready to close off the hose from antifreeze leaking all over the place. From there, you can remove the FITV housing by removing the three bolts (10mm) if i remember correctly. once you remove the housing from the manifold, there is a brass plate on the bottom of the housing that has 2 philips head screws holding it on. remove them and take the plate off. From there, you can see the plunger inside of the FITV housing. Most likely, it is loose and moving around inside. You will have to press the plunger back in firmly and turn to thread it back on. Once it is in as tight as you can get it with your hands, take a flat-head screwdriver and place the tip in the groove on the white ring around the plunger and turn it as if you were tightening it. That will make it secure for use.

Put it all back together, apply coolant lines and then start it up and see if your idle is fixed. If the problem was the FITV and it was as i descibed than i can almost guarantee your idle issue will be corrected.

If it isnt either the IACV or the FITV than look for vacuum lines on the back of the intake manifold for leaks, torn hoses or just plain open lines (not plugged or connected).

If this still doesnt fix your idle problem, try flushing your radiator and replace with fresh coolant. Make sure you do this correctly or else you will not get the results you want, just more headaches. Always replace thermostat when doing a rad flush.

It's okay to quote yourself.