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Hello, my name is Bobby. Thank you for the question. Can you give me the year of the vehicle?
Thank you for the response. If it is running hotter on the highway than it is when it is stopped then you typically have a failing thermostat, a clogged radiator or you have a failing head gasket. You can check for a head gasket leak with a block tester from most auto parts stores. If the coolant is very dirty then suspect a radiator issue. One thing to keep in mind is that the thermostat is the cheapest place to start. If you were going to do anything first it would be this. If you are not sure I would replace this first to see if it will help. Please let me know if you need further assistance.
Did you happen to also install a cooling system pressure cap?
The only other things that I would assume would be the issue would be a very small head gasket leak that will allow combustion pressure into the cooling system and cause this to occur and will not show up during a block test. I would also check the front of the vehicle and make sure that the a/c condenser that sits in front of the radiator is not clogged with any debris. This can cause air flow issues.
I have had this engine fail in a way that will not burn coolant, but instead will only allow combustion pressure to enter the cooling system.
From the symptoms you are having that is what it sounds like. When you are at idle there is not as much internal combustion pressure building up so the pressure cannot bypass wherever the failure is. There are not many other possibilities there unless there is simply a lot of build up in the cooling passages which is not too common.
If it is running at normal temperature with the a/c turned off then It may not be the issue. It should be happening even with the a/c off. Were the upper and lower radiator hoses replaced? Do you have a infrared heat gun for diagnosing further?
Do you have a scan tool to see if the computer is reading the same temperature as the gauge?
If the ignition timing is too advanced this can happen also.
You will either need to see what your scan tool says that the timing is or use a timing light.