Long term fuel trim? 99 Sub 7.4L

What would cause the long term fuel trim values to range between -9% to
-16% but the short term to be +2% to -1%? I'm only getting about 8.5 to 9 MPG.
Any thoughts would be much appreciated.

Obviously, you are reading this from a scanner. If the short term trims are read shortly after the engine is started, or after prolonged idling, the O² sensors could be cooling and going low (artificially lean). The long term trims are averaged over a complete drive cycle, and driving under load could be heating the sensors more, causing a higher/more accurate reading.

You should also check the activity/crosscounts of each O² to see just how sluggish the sensors might be. Similarly, a small exhaust leak can dilute the exhaust gas at low RPM/idle, causing a similar phenomenon.

there is many things that could cause neg fuel trims,like a plugged air filter,a fuel injector dumping excessive fuel in one or more cylinders.excessive fuel pressure,a bad mass air flow sensor.resticted exhaust.a hole in the exhaust.carbon on the throttle body.a bad 02 sensor.seeing that you have a negative fuel trim the vehicle is running rich,it's dumping fuel.the long term fuel trim should be as close to 0 as possible,but -10 to +10 is acceptable but a little high.these are some of the things to check.here is a description of long and short term fuel trims. The long term fuel trim consist of groups of fuel control calibrations or cells. Each cell contains the short and long term fuel injector control values required to maintain the correct air and fuel mixture for a particular long term engine operating condition. The control module monitors the short term fuel trim values in order to determine if the short term fuel trim values are indicating a trend of rich or lean corrections to the fuel control. If the control module determines that the short term fuel trim is able to maintain control and is near 0 percent (128 counts), the long term fuel trim within that particular cell will indicate a value of 0 percent (128 counts). If the control module determines that the fuel system is operating rich, long term fuel trim will compensate for the rich condition by decreasing the injector pulse width, and a long term fuel trim value of less than 0 percent (128 counts) is indicated. If the control module determines that the fuel system is operating lean, long term fuel trim will compensate for the lean condition by increasing the injector pulse width and a long term fuel trim value of more than 0 percent (128 counts) is indicated. If the control module determines that the long term fuel trim values indicate a trend toward a rich or a lean correction within that cell, the control module will switch to the appropriate cell, either rich or lean, and the short term value will return to 0 percent (128 counts).
The short term fuel trim is used by the control module in order to control the short term injector pulse width, or the injector ON time. When the control module is in the Closed Loop mode, the control module monitors the fuel trim oxygen sensors signals in order to determine if the fuel system is operating rich or lean. If the control module determines that no change in fuel delivery is needed in order to maintain correct short term fuel control for a particular engine operating condition, a short term fuel trim scan tool data value of 0 percent (128 counts) is indicated. If the control module determines that the fuel system is operating rich, short term fuel trim will compensate for the rich condition by decreasing the injector pulse width and a short term fuel trim value of less than 0 percent (128 counts) is indicated. If the control module determines that the fuel system is operating lean, short term fuel trim will compensate for the lean condition by increasing the injector pulse width and a short term fuel trim value of more than 0 percent (128 counts) is indicated.
When the control module is in the power enrichment (PE) mode, the short term fuel trim value will indicate 0 percent (128 counts) in order to prevent the fuel trim from compensating for the commanded rich condition of the PE mode.

Brian has posted a very good description of fuel trims and block learn readings.

When you are reading the fuel trims also note if both banks left and right are the same.
And read out all other sensor readings for being up to specs.
If both are the same it is a overall problem.
If uneven it will point to which bank has a problem.
Check fuel pressure and tune end up to start with.
Good Luck
MT

AKA, "block learn" and "integrator" functions of the control system on older ECMs. What scanner are you using? Many handhelds only show an average BLM/long term or INT/short tern, while some of the more functional systems (like EASE) will show data for each cylinder.

Im using AutoTap scanner pluged into my laptop. I took it to the dealer. Bad Fuel pump.