Is The Thermostat To Blame?

[ConfederateTyrant]

Ok, let me update a little bit. It has continued to stay at the very bottom of the normal line, sometimes under it, in the mornings when it's 30-40 degrees outside. However, in the afternoon when it's 50-60, it'll move up slightly (between the yellow line and bottom normal line).

This started about the same time I filled up last, so this tankful suffered at 19.5 mpg (I've been getting 20.5-22 mpg). I filled up at the same pump and did mostly the same driving (not enough difference to bring down 1-3 mpg, especially since I haven't gotten less than 20 mpg in nearly 2 years).

Another observation, when the gauge started staying down, I noticed that my external transmission filter felt lukewarm, when it used to be a lot warmer, and even the valve covers and intake are not nearly as hot as they once were, even after a highway drive.

I'm going to get a replacement thermostat soon and get this fixed, because in all the cold winters I have ever driven, the gauge always went to the middle area, no matter what.

Thanks again for all the advice.

 

[ConfederateTyrant]

Update:
Replaced the Duralast thermostat yesterday and now the needle will go up to the middle mark and then lower and stabilize between the red and pink marks in the picture. The engine is now physically hotter, and my transmission filter is warmer to the touch.

I used a Stant Superstat, because it was a lot cheaper and easier to get than the Motorcraft ones, but if I have to replace this one then I'll get a Motorcraft for sure.

Thanks again everyone.