I've got a 1993 Honda Civic Si, its got a 100k mile D16Z6 and a P28 both are swapped in from my same year EX and the idle in the Si is fluctuating well after the car warms up and even when the fan is on. I've tried cleaning out and taking apart the fitv, the iacv has been replaced, and I don't believe I have any vacuum leaks. When i run the P08 ECU from my buddy's DX it started up and ran fine. Granted the motor was warmed up when i dropped the P08 in and started it, but I'm gonna see if it continues the normal idle after it cools off for ~2 hrs.

 

When I started it off cold it jumped up to 3,200 or so on cold idle and started surging again after a minute or so, but stopped when it came off cold idle unlike the P28

 

Wound up just being a capacitor on my P28 leaked and corroded the board, C18 I believe. replaced all the caps, converted it to a manual computer, and threw it in, started up and after a good 30 seconds it idled fine, never had issues with it since.