nothing worng with your thermostat as u have expalined from the test done. function lang ng thermostat is to immediately warm up the engine such that yung tubig lang sa loob ng engine block ang laman. once it get's to operating temp, the thermostat will open so water can start to circulate as you drive and it will keep open until engine cools down again (pagpatay na).

since hindi naman nababawasan ang tubig & your fans are operating, you may want to try experimenting on a new fan na masmaraming dahon para masmalakas ang buga ng hangin.