it overheats in 20 minutes but no leaks
Hate to say, but the fact that it's a 1997 Pontiac Grand Am, most likely means blown head gasket or intake manifold gasket. These were known for that. Have the cooling system pressure tested to see if there are any internal blown gaskets. Have the thermostat replaced, check that the coolant temp sensor is working and turning on the electronic cooling fan when it is supposed to.
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