My 2009 Hyundai Sonata has recently had a new thermostat installed. The reason for the replacement for the thermostat was the temperature gauge while operating would go from normal to hot. This would happen at any time, any distance. Since thermostat replacement this issue still presists. I am now thinking that it could be something with the radiator. It seems like the coolant doesn't flow? Please help

 

Could be(s),,
defective replacement thermostat
Water pump problem
system air bound
When the engine is cold, you will not see any circulation because the thermostat hasn't opened yet.
If you leave the cap off,and watch the radiator as the engine reaches operating temperature ,when the thermostat opens you should see circulation. Be careful , hot coolant can cause a nasty burn.