here is a good blurb on the cooling system. I know it is talking like the reader is a first timer but it does have some good info on how the lS engine cooling system works. the thermostat is on the coolant return hose but senses engine temp so you need to have things hooked up correctly to sense properly. heater hoses factor into the system.
https://www.pirate4x4.com/tech/billavista/Cooling/