Yet another post in the ongoing saga of my '91...
The truck gets lousy mileage (12 mixed instead of 16) and shows other indications of running too rich. Codes are mostly EGO not switching, left bank lean but occasionally left bank rich. I'm convinced that something, somewhere is fooling the computer into thinking that it's running lean and the computer is forcing it to run rich as a result. Computer, EGO sensors, et cetera checked and / or replaced. The odd thing that the mechanic noticed last time was that his scanner (or whatever you call the thing that talks to the computer) showed that the "Fuel Trim" was at 0 and not moving. He said that this should move up and down (positive and negative) in response to the voltage from the EGO sensor, but for some reason was not.
What could cause the fuel trim to act this way? What can be done about it?