So I bought a 2004 honda civic si HB from someone a few months ago and inside the throttle body they had it taped off. It was idling around 2k before I took the tape off.
When I took the tape off it went from 1800-3k rpm so I decided to replace the IAC. After replacing it went down to 800-2200 rpm but still had the high low rpm. I then replaced the intake manifold gasket. After starting the car it seemed normal for a little bit then when I put a little pressure on the gas to go up to 2500ish it went back to the low high rpms. check engine light showed the iac again after that so I figured I got a bad iac and got another one. after replacing it again the car starts at 2k then slowly creeps up to 4k. Ive unpluged the fi-ecu for 30 seconds and still does the same thing. any help would be nice!

 

I found the rubber intake tube that attaches to the TB starts to rip where it holds on by the worm/ screw clamp. For the longest time I had same problems then one day I noticed a slight tear in the rubber right where it attaches to the TB.

Good luck