Do you think you have purged all the air out of the system?
T-stat location is prime spot for air to collect, which doesn't transfer heat as well to t-stat. so it delays it opening.
I would think after a few heat up cool downs all the air would be out.
Have you checked the rad in the morning(cold engine) to make sure it is full to the top.
If not then you do have air inside and that could indicate a leak somewhere, maybe even in lower t-stat housing, that would put air at the t-stat every time engine cooled off.
The self purging cooling system uses a rad cap with 2 valves, and the fact that coolant(any fluid) expands in volume when heated and then shrinks in volume when cooled.
The larger valve holds the rated pressure, say 14psi, smaller valve in center lets coolant flow back in from overflow tank as engine cools down.
So as engine warms up coolant expands and adds pressure to the system, this increases the boiling point of a fluid and why pressure is used, when pressure gets to 15psi the large valve on the rad cap is pushed open and coolant flows into overflow tank, then valve closes at 14psi.
After you shut down the engine the coolant cools down and volume shrinks until pressure at the cap is -1psi this causes smaller valve in rad cap to open and suck in coolant from overflow tank.
This is why rad should be full to the top, 0 air in there when you check it with engine cold.
If there is air in the system, say after a flush or refill, it will collect at the top of the radiator once t-stat opens, then as pressure builds to above 14psi the AIR will be let out of the big valve first and it will bubble up into the overflow tank and be gone.
As engine cools down coolant will be sucked back in not the air, so system is self purging, IF(big if) there are no leaks.
If there is a leak then as engine cools and coolant shrinks AIR can be pulled into system from that leak point, it is easier to pull that air in than coolant from the long overflow hose and tank.
Or yes you could have a bad t-stat, it happens
Last edited by RonD; 11-12-2015 at 08:08 PM.