I have a 95 Nissan Pathfinder (VG30E 6 cyl.) that under-heats. I changed the thermostat which seemed to be OEM. I checked the new and old in hot water and the OEM seemed OK. I put the new one in and... same problem.
It can't be stuck open twice. I stuck my finger in the radiator then started the car and the coolant started warming in less than 60 second. It's the only test I could think of!!! It appears that it is letting the coolant leak by it. Why would the thermostat leak? I'm the third owner, so I don't know if it was ever changed. I doubt it did this from the time that it was new.