The car is: 1990 Honda Prelude Si (2.1 liter)

My brother's problem is:

I turn my car on in the morning, if I take off when the car is still cold, the rpms stay at their "warmup" level of 1500 or so. Then, all of a sudden the idle will go up and down rapidly from 800 to 1500-1800 rpm repeatedly until the car is at operating temperature. However, it does happen occassionally when the car is warm, but the rpms rise slowly to 1500rpm and then drop down suddenly. Need help!!!!!!!!!!!!!!

 

Could be a number of things. My guess is.. Idle valve is clogged, and needs to be adjusted. I cleaned out the one on my friends 90' Teg. The idle valve im talking about is located underneath the throttle body (where the intake hose connects). It has a small coolant line going to it. Also, has a intake route going from the throttle body to it, but u cant see it unless you take it apart.
I took the whole valve body off the intake manifold (bolts right off), then cleaned it out and turned the white spring loaded **** thingy clockwise (tightened it).

Try it, it only takes about 30 mins at the most. BTW, you might wanna have some extra hose, just in case the old ones split on ya.

 

This may or may not apply to your engine, but on some cars, air in the cooling system can cause that problem. See if this applys to your Lude.

On many Hondas/Acuras, the radiator is NOT the highest point in the cooling system - the cylinder head is. As a result, if there is ANY air in the system it will rise until it is trapped in the head. Also, on many Hondas/Acuras, the temperature sensor that controls the idle speed is right there in the cylinder head also. Since air and water conduct heat differently, the sensor reads hot, cold, hot, cold, and so on as the air bubble bounces around. The engine is literally going back and forth from normal to high idle as this air moves around.

The fix is to bleed the cooling system.