I've put 3 thermostats in my B4000. Same symtoms. I put Ford factory thermostats in every time. I don't know what the deal is, but everything is fine after replacement. They last about 2 years and they hang open I guess?? I have a friend with an Explorer with the same engine and he has had the same problem. The factory thermostat was recommended because some do not have the small vent hole at the top of the thermostat that it
instructs you to put in the up position.
Fishhead