Need Opinions on Rough Idle Cause
 

Hi All,

I have a 2004 Automatic RX-8 with 42000 miles.
The car idles perfectly when the A/C is not on.

However, when I have the A/C on and the compressor starts up, the idle drops from about 820RPM to around 720RPM then jumps back up to about 1200RPM then back to about 820RPM.

Things I've done:
- Checked motor mounts (good)
- Changed coils, wires, and plugs
- Bought ground kit and installed

Now, I have a scan gauge II connected. I noticed that it shows the voltage (I assume from the battery) is at about 14.0v when idle. When the A/C turns on, the voltage dips to about 12.3v and that's when the car has a rough idle.

I bought my battery from O' Reilly's two years ago. It's dated 9/2012.
It has 640CCA at 0 Degress F and 800 Cranking Amps at 32 Degrees F.

Surely there must be something wrong with my electrical system for the voltage to dip like that when the A/C is on, but my battery is fairly new.

Any thoughts? I'd appreciate them :)

Actually, that seems pretty normal to me. Mine did that too when the AC kicked on.

Now it only dipped for a fraction of a second, then jumped up - so if it's longer than that maybe the alternator is going out on you.

Yeah that is just normal to a degree man, but if it gets too bad then you need to have your charging system checked. Also I have had a brand new battery fail after a month that showed 12.8V when checked but when load tested showed an internal fault (POS Optima crap). Also here in Texas with the heat two years is about all you can expect out of a normal battery. But in UT you don't get the extreme heat we get.

Also, if you can, get the codes for the Scangauge II to read stft, and see if it +/- 3% from 0. If it is way off you may have a vacuum leak. I would check your intake as well and clean your MAF.