Forgetting about the time of year for a bit, I agree with what CDX825 said about straight (or pretty close to it in your case) antifreeze not being as efficient at heat transfer. I believe that I've heard this before. There's a reason why it needs to be diluted before using. Until your trip into the mountains, you hadn't driven the truck on the freeway very much (far/long). Your engine is working harder at freeway speeds than it does at slower speeds, around town and short trips on the freeway. I proved this to be true last January when the thermostat in my engine stuck open on my way to San Antonio. My heater worked just fine while on the highway, with temps in the 30's, but it didn't work very well while driving through towns. When your engine is working harder, for longer periods of time, it makes more heat than when it's not working as hard. When the engine makes more heat, the coolant has to carry more heat to the radiator, where it gets transferred to the air, which cools it off. As long as your thermostat is working properly, the outside air temperature doesn't matter as much since the engine's still making the same amount of heat under the same driving conditions. When the coolant doesn't pick up (transfer) as much heat from the heads and cylinders, it can't transfer as much to the outside air through the radiator. Since the heat doesn't get transferred to the coolant as well, more heat stays in your engine's internal parts. Anytime that you shut off your engine, if you watch a mechanical temperature gauge, the needle will rise. This is because the heat is still being transferred from the hot metal parts of the engine to the coolant, but it isn't being transferred to the air through the radiator so it takes longer for the heat to be transferred to the air which is the only thing that actually cools off the engine. This, in turn means that more heat stays in your cooling system for longer periods of time, which raises coolant pressure above what it normally sees while the engine is running. In your case, I believe that what is happening is that there is more than 13 PSI in your cooling system for a while after engine shut off since the heat isn't being transferred to the coolant as quickly. This causes the coolant to be pushed into the overflow tank ("boiling") to relieve the excess pressure. When the coolant cools down enough, it contracts back to it's normal volume and draws the "boiled" coolant back into the cooling system from the overflow. That is why your engine temps don't get over 200* and also why you aren't seeing any coolant loss due to compression gasses getting into your cooling system. No matter how small a head gasket leak is, it won't be a "every once in a while" thing and it will eventually cause a loss in coolant unless the engine is only run for a very short time. PLEASE either replace the thermostat and get the correct coolant dilution in your engine or do some testing like I suggested before you convince yourself that you have a leaking head gasket. Remember that while others have had similar situations as you, no has had your EXACT situation. Other threads can help to point you in the right direction, but they may give you the most direct path to get there.
