04 V8 Sport Premium with 242k miles. Started as my parent’s car at 89 miles. Many of the cooling parts had be replaced over the past 3-5 years: Radiator, degas bottle, thermostat, upper and lower hoses. Last summer I replaced all the remaining plastic parts and water pump. I didn’t replaced the thermostat, although I had one because the nut in the cross pipe was spinning and I just left the existing 3 year old tstat in the car, thinking "it was only 3 years old." The car has run 205-215 on a consistent basis, sometimes hitting the low 220s sitting in traffic on warm days. Tuesday driving home from downtown LA my exterior temp was 112 and my cooling temp was Running mid 220s and up to 229 until I turned on the heater to bring it down to 217-219. Yesterday I was able to get the tstat housing off by gently prying it while turning the nut. I epoxied the nut back in and installed the new Motorcraft tstat and housing I ordered a few months back. After bleeding and driving, the coolant temp was staying UNDER 200!!! At one point it started to get over 210 and I came back home, let it cool, and rebled the system. This morning, we are still running 194-199. I haven't been able to test the system with a longer drive in hotter locations but it appears that the tstat that was replaced in December 2015 by the local dealership was the issue. That doesn't surprise be since this was the same dealership that replaced the degas bottle three months earlier. If you recall, it looked like the picture below in 3 years. I am hoping this concludes my cooling system issues since every part has now been changed.