I have a 2000 Pontiac Grand Am GT. I have replaced the head gasket, intake manifold gasket, water pump and coolant resivour. I am still needing to add about 1 quart of antifreeze about once a month
Have you checked the freeze plugs, heater core, hoses, its got a small leak double check every where
Pressure test the cooling system. Some auto parts stores will loan you what you need to do the tests. Doing this test will force the leak to show its self. Some leaks tend to evaporate before they are visible. With this test you can find them. Also did you change your radiator cap? Is it the correct psi for your car? Bad cap will cause this also. Hope this helps
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