i had the same problem before, take out your thermosat and then try it, its usally just gets stuck closed for some reason, you really dont need it in the summer anyways and with out it the coolint will just keep going round and round, you may wanna put it back on in the winter, but then again i never did.
PS. make sure the car is cold when you do this, very easy to do it's only two #10 bolts.