I have a 1999 honda civic LX. when you first start it it idles normally, then when it warms up for a few mins to a certain temperature i guess, the idle starts playing up and down on its own when the car is in idle. but when i drive it, its normal till i stop.
it is a 5 spd
any ideas?
i just bought the car, so i havent touched it yet, and it has about 80miles on it.

 

My assumption would be that the ECU was just reset. The idle gets pretty screwed up for several hundred miles after the ECU is reset. Give it some time....

 

heres what you can try.

take off your Idle Controll Sensor its located in the back of ur intake manifold. there should be 2 bolts and 2 lines and a eletric plug.

clean it nicely also the back of your intake manifold.
*IF THAT DOESNT WORK*

. check your coolant (sometimes the IAC can idle stupidly cuz it thinks its lacking coolant so it gets confused and starts throwing rev's)so you have the get the air bubbles out.bleed it until u have coolant coming out and not air (bleed it by the thermastat)

*IF THAT DOESNT WORK*

Lower your idling by adjusting the idle screw on the intake manifold (this worked for me)

*and lastly of that doesnt doit*

then that means your idle control sensor is going out.... and you have to replace it. (easy install) if u just go to a local honda dealer and buy a new 1