I just took my new-to-me '94 Explorer on its first backroad trip. It
did GREAT! Gravel roads, fording a foot-deep stream, a steep, muddy,
rutted road- it handled it all. This is just what I wanted.

However, I noticed that on long downhills the temp indicator dropped to
cold. I had noticed that the temperature gauge normally read low, the
needle right at the line between "cold" and "normal" but figured that
could be typical.

I've seen this on other vehicles, and wanted to get it fixed before it
gets cold and I need the defroster/heater.

I figured it was stuck open, but when I got it out (good thing I had a
wobbly adapter and various extensions!) I found it closed.

So I put it in a pan with the new thermostat, fired up the burner, and
watched. The old thermostat opened sooner than the new one, and once
they were opened, I turned off the heat and started adding cold water
to the pan. The new thermostat promptly closed, but not the old one. I
removed them from the pan and the old one was still open slightly,
though it was cool enough to touch the brass part that contains the wax
pellet.

I don't know what goes wrong- if the spring weakens or what, but the
new thermostat works great. There's only two needle widths difference
on the temp gauge between going up a long hill and going down it, and
it reads well into the normal range instead of at the bottom end.
-Paul