to run hot? If so, one guess is that you're not getting all the air out when you fill it. There are many reasons for boiling over. The more description you give, the better responses you'll get. When does it do it? What is temp gauge reading before and during boiling over, compared to normal running position if you have a stock gauge? What makes you think the radiator is bad where the cap is? Did you check the fan clutch? Is the belt tension right?