Thermostat maybe.

[Steve42]

As some know been having a coolant loss for some time now.
But there was never any sign of coolant loss in the engine or drips on the floor.
Well have found it.
Its coming out of the coolant filler cap.
Have now put a new one on.but still the same.
But we have noticed the coolant rises only when the engine fan kicks in at 100c as soon as the temp starts to drop the coolant starts to drop.
Could this be the thomstat sticking or faulty.

Thanks