Talking about my thermostat here. My truck is faaaar from stock so set your OEM thinking caps aside. When outside conditions are just right, like this morning, my primary electric fan will kick on when it's 73 degrees outside. I have a verified 20+ degree temp difference in the cooling system between the sending unit in the head and the sending unit in the front of the manifold (see photo for location). I'm convinced that my cooling system is so effective that under these conditions, (75 degrees outside and vehicle MUST be moving at least 20-30 mph) it's creating a hot side/cold side on the thermostat and it won't open. I'm running a Pro-flo 4 that uses the air gap manifold. Two row all aluminum radiator. Spal dual 11" fans . Dakota Digital fan controller. 180 degree thermostat. Primary fan set to on at 195- off at 190. This morning the fan came on and stayed on the entire 30 miles of freeway driving to work. Guages read 178. Fan controller read 198-200. However, once I slow down to stop and go driving or idle, the Clam...I mean uhhh thermostat, the coolant temp evens out to normal 185-190 on both temp senders. Odd ehh?