Car wants to idle fast at all times, even after it warms up and driven.

What is your question?

The car wants to idle fast even after the car warms up and you take off and drive the car. It wants to run rough going down the road. It wants to idle fast when you slow down and park the car. They are telling me it just runs at high idle all time and they think they smell it getting hot. If anyone has any ideals what this could possible be i appreciate it. I work on cars but i never heard a car doing this. The customer called me yesterday and wants me to fix it for them. Any ideals? Thanks!