If you add coolant to the resevore instead on fill plug will this cause any real damage? Car overheated after adding to the "cool fill line"in the resevore. After researching the manual I followed the proper procedure by removing the fill plug and it worked fine until it warmed up. Could this be caused by air in the system?
Air in the system can cause it to overheat. However, it should draw it into the engine from the overflow. If you refilled the radiator and it is working fine, it should be okay unless you overheated it badly.