My 1993 Prelude DOHC engine up and down. It continues to do this even when it warms up. I just replaced the intake gasket during the long block installation. The engine that was in the car was a H23A1 and I installed a H23A3 long block. Anyone have any ideas on why the idle is up and down up and down?

 

Because you have a vacuum leak.

 

IACV

 

If you had looked in the FAQ's stickied at the top of the page, you would have found this:

https://honda-tech.com/forums/honda-prelude-4/end-all-idle-surge-bounce-etc-problems-thread-1979500/

 

if you need more help after using the FAQ section make another thread