A member of my family just picked up a 91 Prelude H23. Upon start up, the idle will surge up to about 1800 and fall back down to about 600. This does not affect the commute in any obveious way however, once the engine warms up to operating tempature, the idle smooths. Only code I pulled was the speed sensor which we replaced. I'm thinking idle air control valve or vacume leakage but that doesnt sound right considering it it runs fine at operating tempature......

 

Check the coolant level. Low coolant will cause idle fluctuation via the IAC valve or the fast idle thermo valve.

If the radiator is full, check the IACV and FITV to make sure they are not clogged or stuck.

 

Coolant level was what I was thinking, too. Happened to me.

 

I second the motion, its your coolant level. Had the same problem a month ago