Ok, well this has been driving me absolutely nuts for the past few weeks. My idle's been surging between 1500 and 2500 rpms. I took off the FITV today and screwed the white plastic screw shut, hoping this would solve the prob. It didn't. I took off the IACV on the back on the IM and checked that, looked fine, screen wasn't clogged or anything. I have the idle screw on the TB all the way in and even when warmed up, it won't idle anything below 1500rpms.
Now here's the catch and maybe the answer to my probs... I spun a rod bearing in my b18c and replaced that with a JDM b16a. I bought the motor from a local shop and traded my p72 ecu for a chipped p28 to run the b16 with. So I plug the p28 into my car with the b18c still installed, because I had to move the car. The idle was surging with that motor. When I did the b16 swap, I used the GSR TB on the b16 IM because I didn't want to deal with swapping throttle cables and all that crap. So I'm having the same exact problem. I thought it might have been the FITV since that's attached to the GSR TB, but screwing it shut did nothing. I tried resetting the ECU, nothing.

The ECU itself has been socketed for Hondata however... and I'm thinking maybe this has an effect on my idle? I thought socketed meant easy removal of where the chip plugs in, but maybe I'm wrong. The guy who I got the ECU from re-programmed and re-chipped it right in front of me, so I don't think I was given a bad ECU. Any insight would be greatly appreciated... this seriously sucks.

 

So what chip are you running? I had a really bad idle before tune because I was running rich at idle on a base map. Check your AFR or look at your spark plugs. If they are black or dripping wet, then your chip is not tuned.

Also try removing your IACV and turn on the engine. It should surge at 3k rpm but keep steady.

 

I recently did an intake manifold swap on to a friends car and had the same results. The swap was this his car was an 89 HF with a 97 D16Y7 with the HF manifold and throttle body (super small). I swapped a 93 Si manifold and throttle body onto the engine and all was fine. A day later he calls me and had the same problem as yours. I took off the fitv, screwed it in and all was fine util later that day. It ended up being his throttle body gasket had a hairline tear in it and was causing a vacuum leak. He replaced the gasket with a new honda one and hasn't had the problem since. I would check all your bolts for tightness, check all your vacuum lines, and lastly check your gaskets (IM and TB) for leaks. Hope this helps.