the idle speed control valve is VERY prone to failure on zetecs - basically it get all gummed up with oil and much inside and stops working - you have try and clean it with carb cleaner or fit a new one. is is uderneath the inlet manifold, is small and round like C size battery and has a multiplug with 2 wires connected to it, usually on the left hand side of the inlet manifold , right near the main throttle butterfly.