My 96 Grand Am has had several coolant leak problems. I had the water pump replaced and it still leaked coolant, not as much, but still leaking. Mechanic found two bad hoses which he is replacing but he said it also has a crack starting in the radiator. Evidently the cores are plastic? Can I use stop leak on it for a temporary fix? I can't afford a new radiator right now.

I had the some problem on my truck a while back. For a temporary fix you can use an epoxy that you can get at autozone or an auto parts place. just drain your radiator, dry the surface area, scuff the area up with some sand paper, then spread some of the epoxy mix on it. Note that when you get the epoxy it has to be mixed seperate. It comes with epoxy and hardener. Also I would let it set for at least 24 hrs, or recomended on the package to get best results.
Good Luck!!
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You can use stop leak as a temporary fix. You may want to try a wrecking yard for another radiator. Much cheaper than a new one. Try car-part.com for wrecking yards in your area.