have 92 honda prelude si manuel...
after starting up the engine cold the engine revs back and forth....
around 1500 RPM to 3k until the engine warms up then it stops....
can someone tell me whats wrong..??

 

Vacuum leak or the IAC or fast idle valve/solenoid might have gone bad.

 

I had the same problem a while back. I'd proably look into adjusting the screw on the fast idle thermo valve. It's underneath the throttle body, i think theres three screws that hold it on. just unscrew those, then i think you pull it apart, then theres this white plastic screw thing, take an oversized flat tip screw driver or something that can turn it, tighten it down to where it sorta stops i think. refer to a helm's for more detailed instructions.

 

thanks... thought it was going to be a bigger problem...