I finally got to drive the truck in warmer weather and I guess I might have some problems. The temp was upper 80's and after about 15 minutes at 65 mph (3000 RPM) the temp was almost out of the "normal" range. The "ENGINE" light came on. The coolant level is o.k., pressure cap works, etc. I had been using the AC, so I shut that off and slowed to 2800 rpm or so and the light went out. For the rest of the weekend the temp gauge stayed around the "A" or "L" in the word NORMAL on the gauge. Where should the gauge read if the thermostat is good? The rad looks very new on the outside and has a slight amount of buildup on the top of the tubes inside. (Not enough to make me think it's restricted). Since I don't know what type or how old the antifreeze is (it looks new also) I plan to drain and replace it. Is there a recommended brand that works better for the 6.9? I've read about the cavitation issues and will get additives and strips from my local NAPA store, as well as a thermostat if someone thinks my engine shouldn't be running this hot. I'm not even towing and it will be over 90 in a few weeks and I want to be able to use my AC, so I'm hoping I have a bad thermostat and this isn't normal for these trucks. As always, any input will be greatly appreciated. Thanks.