I posted this in the Bronco section but I'm hoping you pickup guys might help as well as your trucks are the same as the Bronco's. I have a 1990 Bronco 5.0FI engine with AOD transmission. About a month ago the high idle while cold stopped working and the check engine light came on and stayed on. When I start it it will idle (barely) until warm and then idles good and even a little high after warm. I checked the codes and the only one is code #51 "Engine coolant temperature (ECT) sensor-signal voltage is too high" I had replaced this sensor, located at the thermostat housing, last spring due to a cracked plug and never had any other problems with that code until recently. I thought that maybe that the sensor had already gone bad and so I changed it again. Still the same code #51. Why would it still show too high voltage, what should the voltage be, and what could be wrong? Do you think this and the lack of high idle when cold is part of this same problem. Gas milage has dropped to about 6mpg around town. HELP!!!
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Did you get this code while it was running? Were your codes 51, 51, 1, 11, 11?

 

No it wasn't running while I checked it. I ran it until warm and shut it off before testing. Code was, I believe 51, 51, 1, 51.