99 Civic RPMs vary while at idle

     1999 Civic EX 5spd 140k miles
At idle, such as at a red light, *sometimes*, maybe 1/3 of the time to 1/2 of the time, the tach will show the RPMS going up a few hundred,
then back down several hundred more. Up and down, up and down... Only when the AC is on. Sometimes the RPMs have dropped significatly, and the engine seems to shake as if starved for gas or air or whatever.
Vaccuum leak in the AC system?
PS... not that it matters, but the AC control unit is on the frizt... this is the dealie next to the stereo, with the 3 knobs that control a) Which Vent, B) Temperature, C) Fan Speed. During the fall, I started having trouble with it, then in the winter all the fan speed knob stopped working. Temps have warmed up with Spring at hand, it works better now. I'll think about replacing it during the fall unless it dies in the summer. Mentioning this...just in case....
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On Sun, 11 Mar 2007 21:00:58 -0500, the phelper wrote:

Off the top of my head, two things came to mind: You have a vacuum problem, like you mentioned. Your Air Flow sensor needs cleaning.
Look for vacuum leaks or a stoppage, and you can try to cleanthe AF sensor with carb cleaner or Lectro-motive (keep away from rubber or plastic).
But, since you mention the AC problem, try turning the HVAC system OFF totally and see if it goes away. If the compressor is off, then when the system turns on, it may be loading the system too much and causing your problem.
As far as the fan control, these are controlled by a resistor pack, usually located in the fan or behind the control; it looks like three or four little springs. Does it work on some speeds but not others?
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Hachiroku $B%O%A%m%/ wrote:

honda don't have an air flow sensor. they have two air pressure sensors and they measure the differential between them. no cleaning required.

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On Sun, 11 Mar 2007 19:48:39 -0700, jim beam

Thanks Guys. Yep, it only happens with the AC on.
I don't know thing one of checking for leaks. Any ideas what that should run costwise?
Thanks again.
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