I have a 2003 Santa Fe. A couple of weeks ago my temp gauge began reading lots lower than it has since I owned the vehicle. I've noted that my gas mileage has also decreased a great deal. I seem to recall that when a vehicle is first started the engine receives a rich mixture of fuel to enable the engine to run smoothly. I'm thinking that my Santa Fe is operating under this condition. In the past I was using less than 1/4 tank of gas for a 120 mile trip. Now I am using very near a 1/2 tank for the same trip.
On my temp gauge the tick marks indicate 1/4, 1/2, 3/4, and needle pegged in red zone. In the past the gauge indicated almost right on at 1/2. Now it indicates very near 1/4. In the past the needle was very stable right near the 1/2 tick. I would guess the the needle rests about 1 needle width below the 1/2 tick. Now it only goes to about 1 needle width above the 1/4 tick.
I know this is not normal. What I don't know is what causes this behavior.