my friend at work has a prelude its eathier a 90 or 91 2.0 si and it has a verry bad idiling prolbem.. it goes up and down from 1500 to 3000 rmp and wont stay stedy just keeps going back and forth. i sugested mabey a bad sensor or idle speed control.. anyone have any sugestions>??

 

Might be a vaccum leak. Check the vaccum hoses on the intake manifold to see if any are loose or cracked. If that's not it, I'd start checking the sensors.