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What is the Year, Make and Model? Have you had the coolant flushed lately?
The first thing I would check is your radiator cap, it is designed to maintain a constant pressure in the cooling system. The radiator cap allows expanding coolant to go to the coolant recovery tank. When the fluid has cooled, coolant is drawn back into the radiator through a vacuum valve in the cap.
I would next check for air in the system. You should have a small bleeder valve on your upper radiator hose, with the vehicle running crack this valve to let any air out, you should start to see a steady stream of coolant.
I would then look at the thermostat. It is the brain of the cooling system. The thermostat should be changed out about every two years.
If all these are good I would consider having a compression test done to see if there is a bigger issue going on like a blown head gasket. Hopefully that is not the case.
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