bobbyluv35

2002 cavalier base model. this car had no issues. ran like a top. drove great. as the weather got colder, i wanted to make sure antifreeze mixture was correct. ever since i checked it it has overheated every time i try to fix it. i noticed the cap looks worn and smaller in diameter and much flatter than new one. there is no radius in the o-ring. its flat in the groove. is there anything else i should look for