Not 100% sure on a 2008 4/4 but later models have the full amount of coolant circulating all the time. This because the heater core is permanently fed, with dash cabin heat control operating flaps in heater/air box which adjusts the mix.
Thanks for that, I wasn't certain when they became a single circuit, even so, if you switch your heater fan on when the ambient temp goes cooler at night you will still a drop in your water temperature, Same effect, as when water temp rises in stationary traffic, switch your fan heater on and the water temp falls.as you have increased your effective fan water cooling operation capacity.
What I was trying to isolate obviously not very well, was whether as it was colder at night when driving the car was he putting the heater on, which would cool the water temperature he was experiencing. I also wonder how reliable the instrument readout is and whether he has a drain on his electrical circuits at night eg all the lights on, heater on which may be influencing his instrument readouts. I know it shouldn't do, but we are talking Morgans exposed wiring and poor earthing contacts, condensation in instruments that tend to dry out when the lights are on all could be an influence , if the obvious is not the issue. etc.
