Interestingly, when driving at a maintained speed, the engine should run its coolest simply because there is a continuation of airflow through the radiator just because the vehicle is moving.
Tell me, have you noticed any coolant leaks? Is the coolant level full? Also, have you noticed any white smoke or creamy substance mixing with the engine oil?
Here is a link that I want you to look through. It explains how to check for a bad head gasket. I'm not suggesting that is the problem, but I want you to check a few of the simple things listed. For example, is there white smoke from the exhaust? Also, the link shows what oil looks like when contaminated with engine coolant.
However, before you do this, simply make sure the coolant level isn't low. That can cause this problem as well. (specifically the heat issue).
Here is a link that explains how a cooling system works. I thought it may be of interest to you.
Let me know if you are able to find anything that I mentioned. Also, please feel free to ask questions if you have them.
Tuesday, November 10th, 2020 AT 12:03 PM