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my car use to idle closer to the 800 mark before the whole carbon build up. Now it idles at exactly where you're talking about. I noticed the low idle when my car started sounding like it was going to die every once in awhile at stops.
The idle will change depending on the load on the engine and when the A/C is on. When there is no load (in drive) and the A/C is off you should see 550 to 600 rpm. When there is a load and/or the A/C is on you should see a couple hundred more.