Have had same problem. It is the thermistor type coolant sensor located next to the water temp sensor. Has 2 wires coming out of it. What it does, at start up, the computer system is an open loop, this sensor sends info to the computer about water temp which sets the fuel richness and ignition timing, info received at computer, loop closes and engine keeps running.
Took me a week of asking questions and reading about Ford computerized engines to understand the system, went to that sensor, started to pull wires off,the plastic head and 10 feet of fine wire pulled out of the case. Bought a new one at NAPA, installed, ran perfect. My s-65 is a 1999, Ford LRG- 425 EFI engine, sending Unit- water temperature (Ford part No. F7DZ-10884-AA NAPA ECHLIN TS4056 In my parts search I also show Motocraft SW5174, not sure about this number, bought the Echlin, have box and bad part in front of me.
Bob Southern Ohio