Mine Idles at 600 rpm, I only notice the headlights dim when ABS kicks on. I had my alternator bench tested for free... there was a squeaking problem but it wasn't the alternator which passed flawlessly.
When the truck is off I get 11v, when it is running it is at 13v but that is at startup which is around 1100 rpm. In the tuning software I can see that if the system does not have enough voltage it will bump up the rpm to charge the battery....
However my truck would never idle at 600 with the stock tune.
Maybe your IAC valve is bad?
98 XLT "Off Road" 4x4, Gutless 4.0 Automatic, 4.11 265/75/16, Supercab, SCT Tuned
Last edited by leadfoot; 12-12-2009 at 08:04 PM.