I swapped a JDM F23 into my friend's 2000 accord EX 5 spd. Symptoms:
1) Starts and idles fine.
2) Idle beings to fluctuate between 1000 RPM and 1800 RPM
3) Immediately upon setting P0505 code, idle drops to fluctuating between 1000 and 1200 RPM
4) If I plug the IACV port in throttle body, the idle returns to a smooth normal

I have cleaned the IACV and always swapped with the JDM.
I did not swap the jdm CPS sensor out with original, that is all I can think, of, but they looked identical and it idles fine sometimes, so I suspect this is not the issues.

idea?

 

It sounds like you might have a vacuum leak. Pick up some carb cleaner and gently spray around the intake area and see if the idle changes, especially if you removed the intake manifold. If it does, you've found the leak. If the jdm engine had a crank sensor already attached it should be fine.

 

Thanks - Forgot to mention that already tested for vacuum leaks as well.

 

Fine it was a vacuum leak! Back side of upper intake to lower intake. Thank guys.