Erratic idle

this a 91 honda civic h/b w/ 1.5L engine. i can hear the car idle up an down when sitting at a light.Does not do it all the time.Could it be the the fast idle thermo valve?

could very well be that. or it could be others....low coolant, IACV, and so on. could you give us some other symptoms?

does it do it as soon as you start the car? after the car warms up? sometimes, it comes and goes? anything other that doesn't seem right? car get hot or stays normal? would you like me to shut-up now, or shall i ramble on?

check vacuum hoses, if they are ok, adjust your idle screw and see if that helps. Thats all I can think of.

when the car is warmed up an your sitting at a light or what not i can hear it surge up an down, i tried to mess with the idle but this did not help.

sometimes when you take off the map sensor it makes that same kind of idle goes up then down up and down so on and so forth check that t.p.s!