Erratic Idle when A/C turns on.

**RAyala2.7T** · 06-15-2008 04:50 PM

This problem gets worse as the outside temp. goes up. Vacuum will drop to 18in Hg when the a/c on which is normal, but as soon as you hear the fan come on the rpms drop and rise from 790-950rpm. when it does hold a stable idle, the vacuum fluctuates between 17 and 18inHg.

With the a/c off though, vacuum is 21inHg and holds nice and steady and my idle is near perfect save for the random hiccup every now and then.

Has anyone heard or had this problem with their engine with the a/c on? does your car act anything like this with your a/c on?

**t1demont1** · 06-15-2008 05:17 PM

my vaccum does drop a few psi with the ac on, its normal. how often will the rpm drop and raise from 790-950?

**RAyala2.7T** · 06-15-2008 08:24 PM

^^^As soon as the fan turns on it will start bouncing from 750-950 pretty fast and sometimes it won't even stop unless i turn off the a/c.

**ChescoS6** · 06-15-2008 08:33 PM

my car does the exact same thing. I would be interested in knowing the cause also.

**djwimbo** · 06-15-2008 08:36 PM

that's normal. As the clutch cycles it'll drop the rpms down. The A/C fan pulls a lot of amperage on startup, which will cause your alternator to go ape shit and slow the engine down as well.
Combo these two, you get what you have. This is normal. Don't worry about it.

... Now do it w/ Street Density Mounts. Everything in the car goes nuts w/ the A/C on.