Hey I just put a 98 base engine into my 99 shell, atm she starts and drives but my idle is going up to about 2000rpm and droving and doing it over and over. I suppected coolant is trying to be sucked in and possibly I have air bubbles. I rechecked all hoes vaccum lines and everything else but can't seem to find any unhooked hoses or plugs. I was also told that I may need to bleed it, and or flush, and change out the termostat. I'm gonna do the thermostat later tonight but wonder if any opinons or answers to where else to start or how to fix the problem will be appreciated thanks

 

If it does it when its cold then adjust the FITV, if it does it all the time replace the IACV. While there is a CEL for the IACV it only comes on if the engine doesn't detect one, wont throw the code if it's going bad.

 

This should give you all the help you need.

https://honda-tech.com/forums/honda-prelude-4/end-all-idle-surge-bounce-etc-problems-thread-1979500/

 

mine did it when the line going from the valve cover to the intake wasn't there. I replaced it and it's fixed. Clean your IACV as well

 

I would start by cleaning the IACV, it's free and quick.