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....

 

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?

 

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

 

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.