Melodyftw

Hello. I have 99 civic EX, bought it from mom and pop dealer about 2~3 months ago. They turned car on even before we arrived so we didn't notice anything when we purchased the car. But ever since car idles weird.
Because of that my husband changed fuel filter, cleaned throttle body, cleaned IAC valve and cleaned map sensor.
But it still idles up and down. In the morning when I start the car, it does not go up and down but when I drive around and park the car, go to shopping and come back the car and start it, it raves. Weirdly, some days are totally fine, like yesterday and today, it was around 70~80 degree here in Texas.
Oh and one more thing, he says it seems like air goes in radiator because coolant in reservoir tank goes really really up high like it's going to spill out.
Would it cause unstable idle?
What should we do?

ezone

Surging idle; could be many things such as a sticking IAC valve or vacuum leaks. Someone could have jacked with the TPS or the throttle stop too. Or even backed out the air bypass screw on the throttle body, that is supposed to be for setting base idle. Too many things to guess from here.

Pumping the cooling system full of "air" and sending all the antifreeze to the reservoir: Probably a blown head gasket. Needs further testing.

HTH