I can do better than that Rainman. I can post the page about fuel trims here. This is fresh out of the Snap-On Scanner Jeep Reference Manual.
Obviously, you have two Fuel Trims.. You have your long term trim, which is the adaptive gain... this is what your XJ has learned over many cycles.. You also have your short term trim. Its what the Jeep is doing currently.
The ranges are from 0-255. 128 being the middle range and where your Jeep is running at the perfect 14.7/1 air/fuel ratio.
Anything below 128, and your fuel injection system is metering less fuel. Anything above 128 and your fuel system is metering more fuel.
All this only operates in closed loop, when your engine and O2 sensor are warm. In open loop, which is a cold engine, declaration and acceleration, the default setting is 128.
This explanation from:
https://www.aa1car.com/library/what_is_fuel_trim.htm
"POSITIVE fuel trim values mean the engine computer is adding fuel (increasing the pulse width or on-time of the fuel injectors) to add more fuel to the engine. In other words, it is attempting to RICHEN the fuel mixture because it thinks the engine's air/fuel mixture is running too lean.
NEGATIVE (-) fuel trim values mean the engine computer is subtracting fuel (decreasing the pulse width or on-time of the fuel injectors) to reduce the amount of fuel injected into the engine. This is done to LEAN out the fuel mixture to compensate for what it
perceives as a rich running condition".
So, Rainman, your system is trying to compensate for what it perceives is a rich running conditon, by trying to lean out the mixture.
According to the website, rich fuel mixtures generate negative fuel trim values and the problems can be:
Leaky fuel injector
Excessive fuel pressure due to bad fuel pressure regulator or restricted fuel return line
Extremely dirty air filter or restrictions in air intake system
Exhaust restrictions (clogged converter, crushed exhaust pipe or plugged muffler)
Bad O2 sensor (output shorted to voltage so it reads RICH all the time)
Its interesting putting all the pieces of the puzzle together. Obviously, Jeep didn't want us to know all this info back in the day. Thats why only they had the Jeep scanner. Now, its easy for us to figure out what is going on, using the right tools..
Cheers,