Yeah, do a reverse flush. You can use the method I found using vinegar and lemon juice, blow out all the coolant with compressed air first, then fill the thing with the vinegar lemon juice, let it sit, the longer the better, blow it out and repeat, each time I did my son's (3 times) we got more crap out of it. Now it makes heat again! I drove it for two years without and finally had it with the thing, so, Good Luck! Changing the core is a major PITA!!!!!