I think you should idle at 650 rpm +/-..thats what I experienced when the car was "normal". I have a big problem with Nissan dealers in general. They are bad at diagnosing problems, and like to replace parts until they finally get it right, causing a lot of expense. They also don't "fix" something, but replace the whole unit. I had a cracked turn signal lens once and instead of replacing the lens, they wanted to replace the whole light unit ($120.00). I also had a bad pump in the transmission of a Pathfinder, and they wanted to replace the transmission, radiator, and something else, for $6,500 "we dont fix transmissions, we replace them". I got a transmission shop to rebuild it for $1,800, new parts and labor.