If it does not overheat on the highway but it does at lower speeds, it's due to an inoperative electric radiator fan. It's not needed with the natural air flow at higher speeds.
Bubbling at the radiator with the cap off is not a good indicator of overheating. The coolant can get up to 220 degrees. That's eight degrees above boiling for the water in the coolant so it can bubble. The radiator cap keeps up to 15 pounds of pressure on the system. That raises the water's boiling point to 257 degrees.
If the temperature gauge on the dash indicates the engine is running too hot, first check to see if the fan is turning on. If it is not, start with diagnosing that circuit. If it is running, check for bubbles in the reservoir. Bubbles suggest a leaking head gasket.
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Saturday, December 5th, 2020 AT 9:58 AM
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