Hyundai,Car heat is cold until I add water and over heating with out telling me

Hi my Hyundai Santa fe 2002 heat is going cold then it goes hot when I add water to it then it goes hot and I think my weeping hole is leaking out the water pump when I'm in park or sitting more then anything but when I pit it in air it is cold not hot like my other cars have some when they over heated could all this be because some one left my overflow cap off and my radiator cap not working all the way or do I have major problems due to the over flow cap being lost with out me knowing and I drove it I put in a sealant for the radiator and block heads heating core and so on.. It seem