So my 2000 civic HX with a y8 swap has been giving me trouble for the past few days it started a few weeks ago I went outside to leave for work and it idled up to 2000rpm then dropped down to 1500 and sat there if I gave it gas it would idle back down to 1500 but it would jump to 2000 and go back and forth between the two I replaced the IACV and adjusted the the throttle cable bc it felt like I could pull the throttle blade close when it was at a idle and it would idle down that solved it the problem for about 2-3 weeks and now it’s back I had a coolant leak on the thermostat housing I fixed that bc I Hurd about air in the system could upset the ects I cleaned any buildup off the inside of thermostat housing and the end of the ects then I bled the system and the problem is still there also the weird part is if I unplug the iacv and plug it back in the idle goes normal but if I give it gas it sits at 1500 and jumps like there a vacuum leak and if I go for a drive it will idle completely fine and drive fine too if I shut if off and turn it back on it will idle completely fine but I shut it off and let it cool off it will do the idle issue again have no idea what’s happening please help me

 

High idle would seriously make me consider it has a vacuum leak, I'd find a friend with a smoke machine and test for vacuum leaks. I myself with the D series usually found the intake manifold gasket to cause issues (Have fun if its an OEM intake manifold gasket, all the ones ive replaced have been baked on).

If you get it smoke tested and confirm theres no vacuum leaks, maybe try re-bleeding the system? Sometimes they can be stubborn and take a few different times bleeding it, make sure you pop the coolant bleed screw to ensure theres no air there (Fill until coolant comes out as a steady stream)