Personally, I would change the thermostat. A 180Ā° thermostat as Joe pointed out, should be closed to heat the engine up then be able to open or close to maintain that temperature. Engines are more efficient at 180Ā°. If itās dropping to 160Ā°, itās not working properly.
My thermostat was working perfectly then decided to not open for ten to fifteen minutes. Once it opened it did maintain 180Ā°. I was concerned that if it took so long to open, it might get stuck in that position. I changed it out and now the gauge starts to move in about five minutes.
