remove thermostat. Refill with plain water and run engine. Stick a turkey or cooking thermometer into radiator neck and let idle. Watch for water flow and temperature climb until it stabilizes. Shut down if it climbs past 195 while idiling. Could be stuck thermostat, pinched hose, slipping belt, bad waterpump, wrong head gasket, etc.
Do a compression test. Pull head only as last resort.
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