90 prelude si 2.1,idle fast then slow

I had a 92 honda prelude with a rebuilt engine, the original owner put all the vacuum hoses in a mess, and connected things to weird places. The engine would have fluctuating RPM, idle slow, then fast, speed up then slow down.

It took us a while to figure out why, but in the end, all we needed to do was remove all the vacuum lines and connect them properly. If you have a repair manual make sure you look through it and that all your vacuum lines are connected correctly.

Before that we also had to fix up a few electrical wiring mistakes and errors (that I believe was a different problem on its own)

I hope this helps, did you take pictures of the old engine before you took it apart? so that you could see how everything goes back together?

It's also good to re-check all of your connectors, if you have a sensor not connected, or 2 similar sensors connected to the wrong sensor you could get the same result.

If you don't have a repair manual, it's a good idea to purchase one. they are rather cheap. dealers sell them, in Canada we can also buy them at Canadian Tire (I'm not sure where you live, so I don't know where you would go to get one other than a dealer)

I hope this helps, keep me posted, and have patience it can be frustrating