Just a question regarding Vacuum

Is it common to have the intake vacuum drop when the radiator fan or condensor kick on while idling. I'm still trying to find the culpritt to my 99, 3.8 get poor mileage. I checked the vacuum this weekend and it was right at 20hg (spec. 17-22) but would drop to about 17hg when either of the above would kick on.

It's normal to loose some vacuum when a high electrical load item kicks in. The demand on the alternator will require more power from the engine, forcing the idle air control to bypass some air around the throttle plate to maintain idle speed. Basically the IAC causes the vacuum fluctuation.

Joel