I noticed that the temp gauge on my 02 5.4 is only coming up to about 1/4 scale. I checked the temp of the thermostat housing with my infrared thermometer and it was low. So, I looked on rockauto and autozone web sites and autozone has a failsafe thermostat. I think that may be the one to use. None of the many choices on rockauto claimed to be fail safe that I noticed. What have you guys been using?

 

I'd use an OEM Motorcraft or a Stant. A failsafe locks open when it overheats, you have the opposite issue.

 

Motorcraft!!.......also might check that your fan clutch actually slips...

 

Motorcraft may be the best thermostats but I have experience with two of them failing closed. One was in my 95 Mark VIII Lincoln. I noticed it before the engine was damaged and was able to let it cool off and drive about a mile before stopping to cool a couple of times before it decided to open up again and I was able to drive it the remainder of the 5 miles back home. The other one was in the 95 Mustang V6 that I bought for my son. And of course the 3.8 V6 blew a head gasket like so many of them did.