What can cause revving after starting honda prelude 90? It just keeps revving up and down.

Sounds Like you may have a trans issue. If it's revving while shifting (called 'flaring") and/or downshifting harshly coming to stops. It's likely internal failure of clutchpacks/bearings which causes contaminants of metal and clutch material to be dispersed throughout trans. including the very precision valve bodies. Valves begin to stick, etc. And you now have problems. Have it diagnosed at Honda dealer, or reputable trans shop.

Check that the battery connections are tight.

Is it revving up when the car is idling? Is the idle speed higher than it should be? If so, you either have a vaccum leak, or your idle air control valve is bad. Sounds like the second, but i'd take it by autozone and have them pull the check engine light codes to be sure. They do it for free. If they say its a vaccum leak, let me know and I'll explain how to find it.

I had a 90 prelude, the code on mine was on the glove box if when opened it was on the outside of the left side when facing the glove box? not sure if all are the same??

Maybe the crank speed sensor is faulty - would confuse ECU and show wierd rev counter results. Any engine check-light (MIL) or stored error codes?

motor will not turn over at all? well sounds like one of two things
1 you broke the timing belt and the pistons hit the valves sticking one thru a piston, thats why it wont turn over
2 you have a wiring problem keeping enough voltage from getting to your starter

maybe the O2 sensor

check for electric short.
if you have factory or after market alarm, that may be the cause.

pistons in brake r sticking need to b stripped n cleaned

This possible solution is also weird. I have a '91 Prelude Si also- GREAT car. Do you have anti-lock brake option? If you do, check the fluid level in the anti-lock reservoir. My Prelude did this revving thing twice, and while checking under the hood to see what the heck was going on, I discovered the brake fluid level was very low. I topped it off, and after giving up trying to figure what was wrong, I restarted the car, and no revving problem. I don't know why this is, but it worked for me twice. Countrycurt0