Temp Issues in High Country
So I left from the coast and into the High Sierra's today with my 97 D90. When I was in the 5k foot (altitude) range I noticed my temp gauge plummeted to the very bottom (exactly what it looks like when you start it in the morning). Obviously alarmed I pulled over within minutes and checked all fluids, coolant, oil, trans, master cylinder, etc..., all was where it should be but the engine was very cool to the touch. In fact when I opened the radiator cap it hissed for a second but not to the extent that the engine was really hot and potentially out of coolant which is why the gauge could have read COLD. In the last couple of hours the gauge will get to the 1/4 mark but never to the 1/2 mark where it typically runs.
Thermostat is fairly new
outside tem is 30 or colder
altitude is 5k or greater
truck is serviced regularly and all up to date
What do you guys think, what is wrong with my engine temp or gauge, or is it just the area I am in... Thanks in advance!!