Thanks for the reply...yet there is ample evidence that my thermostat is working properly. The motor gets up to temp and holds around the 160 mark.
I have discovered for sure there is a tube that protrudes from the overflow cap. And I believe the reason the coolant level remains at a lower than normal level is because of the missing tube in the overflow tank. When the system heats up, it expands into the overflow tank, and is unable to return because of the missing tube. Therefore, the coolant level will always have a new normal level as it will no longer be at a level to get pushed out into the overflow tank.
So this should explain why I only see the (problem) temp rise when the outdoor temp is warmer in combination with the low coolant level.
I was able to get to the dealership to verify the missing part, and purchased the correct tube.
