Odd idling issue

We just finished replacing the headgaskets on a friends 35th Anniv. Ed. and got it running pretty good. Everything seems to be fine, engine runs smoothly and drives Ok but we noticed something strange with the idle. When the headlights are turned on the engine idles up a couple of hundred rpm and when the headlights are turned off it idles back down to where it was. We're talking 700-900 rpm. This while in park. We didn't check it in gear but are wondering why it does this. We never noticed it before. Has anybody else seen this or know why it would react this way? It's not a real problem, just a curiosity at this point.

Sounds good

That sounds about right actually.

May depend on how full of a charge the battery has on it as well. Any time the engine electrical system "feels" a larger draw on the battery ECM should raise the rpm's to compensate for the extra load.

Same thing with A/C coming on.

Mine does the same thing when it used to idle lower han 900 rpm's.

Smitty