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3 Answers
Coolant full? All air out of system? Coolant temperature sensor bad? Any check engine codes? Have the system pressure tested for leaks? Does it hold pressure (your specific pounds of pressure for your make and model). Thermostat installed correctly? Water pump bearings smooth and leak free?
Is coolant new or mostly old (over 5 years old). Did you use premixed or mix yourself? Did you use distilled water for mixture?
Bad cap on radiator? Does it hold pressure? (Generally listed on cap)