what would cause a car to overheat after until take thermostat out

Did you test the new thermostat? Just cause it’s new doesn’t mean you good. You should also flow test the radiator, it could be a combination of the two causing the overheat.

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Maybe you got a defective thermostat, unfortunately it happens sometimes. Or maybe you put the thermostat in upside down. Or maybe you didn't bleed the air out of the cooling system. Hope that helps! Jim

If the engine overheats with the thermostat in, but not when it's out, you simply have a defective thermostat that needs to be replaced. If this is a new thermostat, it's likely you are installing it backward. They have a flow direction.