02 Jetta 1.8T getting hot only in hot weather
 

Hello all, I have a '02 1.8T Jetta and it runs hot, but only on hot summer days. Cooler days (under 90) and in the winter the car runs fine. Would this be a thermostat issue? Or more of a sensor issue? And yes it has plenty of coolant without any coolant leaks, and gauge is always were it is supposed to be without averaging at all. Although it does "use" some coolant from time to time (i.e. gets low) hot rather hot days. But I can not see where I have lost any coolant from a leak. Any idea? Thanks for all comments..