I've got a 2006 Mustang GT that runs hot. Coolant boiled completely out of the reservoir with cap on but temp hand stays at halfway mark unless I continue to drive it. Installed new thermostat and water pump and drained all coolant but I’ve been using water since to flush and prevent wasting coolant. It did the same thing so I installed a new cylinder head temp sensor. I drove it last night and it ran hot and water boiled out of the reservoir while the cap was on again. I waited about 30 min to drive again but I only drove 60 all the way home (15 min) and car didn’t run hot but once I got home problem was still there. I took the cap off and let it run for about 10, 15 more minutes. Water still boiled out but temp hand stayed at halfway mark. I noticed since I bought the car once my fan turns on, my car seems to die down a lil like a lot of current is used then it goes back to normal.