It could be the throttle potentiometer, the throttle body or the link between them. Since youve checked the potentiometer, probably the next thing to look as is the throttle body, take off air filter housing and have a look around particularly anything that looks like a stepper motor round housing on side of throttle. any loose wires, check the throttle moves and isnt stuck. Dont have 1.6 petrols in uk and manuals are aloof, so can only guess.