when the A/C is turened on the engine needs to be idled up, the valve with the white screw will make the adjustment, as long as the electric switch controling it is functioning correctly. The switch is mounted on the top forward part of the engine valve cover and will have two small vacume lines going to the valve with the white screw head. The power stering idle control switch is screwd into the pump and has two medium size vacume lines attached to it, this valve also will idel up the engine when you are turning the wheels.