i have a 1997 mercury villager gs van. the temp. gauge went to h. i changed the thermostat added the right amount of coolant. when i took it out for a test drive the gauge went all the way to h again. there was no heat before or after i changed the thermostat.
heating on 1997 Mercury Villager
The 3.0 lt nissan engines are hard to remove air pockets from the cooling system. I use an air lift system for refilling, which puts the system under a vacuum before filling with coolant. You can try filling then running till operatinng temp, leting it fully cool and repeat over and over.