another idle issue...sorry

ok i am new to the civic forum, so please be gentle. I have an 89 civic,1.5, auto with air. I couldnt find this specific problem. The idle is fine engine warm or cold, but as soon as i turn on the A/C the idle starts to rev up and down. Not crazy high or any thing, but enough to notice. And when I turn the a/c off, the idle returns to normal. No codes on the ecu, any thoughts. The Haynes dosent really have anything for this problem, so any help would be great.

My first guess would be IACV - Idle Air Control Valve. It's on the back of the motor and regulates the idle. Since your ecu revs the motor up a little when the a/c is on the iacv has to adjust to that, and usually if it starts going up and down fairly quick it's because the adjustment gets to a point that it's out of the iacv's range and it can't compensate. At this point it just goes up and down trying to find the correct adjustment point, but never finds it. Might try adjusting your idle screw down a little.

It shouldn't be the IACV since he has no issues normally, sounds like the PCM is having issues adjusting with the A/C on

Ya, I just usually prefer to adjust stuff first before replacing parts.

Same here, but it doesn't sound like a IAC problem, if it was, it would be all the time. Just got done fixing mine. Sounds like its possible the PCM isn't picking up that the A/C is on, or just isn't adjusting correctly for it. Couldn't really tell you a whole lot about it because I don't have A/C.

i sort of agree with the 2nd post, just for different reason. ac takes power from engine, engine is old, pcm has to adjust by adding more power to the engine, my guess would be - by sending more air/fuel into it, revs go up, ac turns off - it all goes to normal, and so it goes.
i don't think there's anything wrong with the car. it's called AGE. i just turn the screw on idle limiter to jag it up higher some.

actually, general tune up should help in itself.

ok i will try all of that and report back, i have a spare ecu, so i will try it. And i will check the idle on the engine as well.

ok i will try all of that and report back, i have a spare ecu, so i will try it. And i will check the idle on the engine as well.

yeah, but it's not ecu. ecu is just trying to prevent engine from stalling. i think, it's just lack of power due to advanced age. that's why i said - maintenance/tune up.