changed radiator n thermostat. Now problems bleeding air.Thermostat sticks shut on 2002 Mercury Cougar

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Thermostat sticks until I burp it. Hear bubbles in lines n motor. Added water behind then replaced thermostat. Could that be the cause why it won't bleed?
(2) Answers
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How bad did it overheat before the radiator and t-stat were replaced?
It got hot but has no symptoms of a bad head other than air bubbles and a thermostat that sticks every 300 miles.

Other than that it runs good. Doesnt get hot till thermostat sticks.
Thermostat 'sticking' because of air pocket. t-stat won't open until hot liquid hits it steam/hot air won't do it. Have it tested for a blown head gasket. Kits to test for combustion gases coming through cooling system are available at parts stores.
i have a 2002 mercury cougar the belt keeps slipping off and the other day i was at walmarts and it made a weird noise and started smoking well i thought the belt had just came off again.......we were puttin the belt back on and noticed it had gotten so hot it put a crack in the radiator so im now in need of a radiator im curious what other make and models of cars will interchange with my mercury cougar?