In Reply to: spoke to soon posted by lonestarjeff on August 08, 2018 at 23:01:23:
You have a slight head gasket leak. High speeds/ac/trailering build higher pressures in the engine, the heat opens the gasket slightly, and you're getting hot gasses directly into the coolant. A block tester will confirm the gasses in the coolant.
It's not unusual for the 4.6 of that vintage.