all liquid cooled engines develope pressure after warm up. Our Treps are designed to have no air in the system. When filling the system you must open the bleeder valve located near where the upper radiater hose is connected to the engine. After filling and bleeding any remaining air will be removed at the de-aeration bottle (the bottle with the pressure cap on top) as long as the bottle is filled with coolant up to the fill line. Your trouble of overheating at idle but not on freeway, with both fans functioning can be caused by a few different things. Including :Low coolant level (air in system),dead bugs and other assorted filth in the fins of A/C condenser core (in front of radiator), water pump,clogged coolant passages in radiator, thermostat,. I put these things in order of most likely to least likely in my opinion. note:when thermostats fail they usually are stuck open causing engine to run cold. water pumps normally don't fail to pump coolant, they like to leak and become noisy. If the pump is leaking your coolant level will become low and you'll have air in the system. If the pump on a 2.7 is leaking it is a very bad thing. That coolant will get into your engine oil and render your oil useless and cause lots of damage to your bearings and break your timing chain. Check your engine oil ,if it looks milky or creamy and or has an unusual aroma, then you got coolant in your oil. In this case fix it on the spot or have it towed to a shop.