This could indicate a vacuum leak, or perhaps a defective IAC.

For an Expedition, a common cause of the P0171 and P0174 codes is a
contaminated MAF sensor. I'd try cleaning the MAF sensor. See:

http://www.fordf150.net/howto/clean-maf-mass-airflow-sensor.php
http://autotechrepair.suite101.com/article.cfm/ford_mass_air_flow_sensors
http://www.tccoa.com/articles/intake/mafclean.html
http://www.berrysweb.com/clean_your_maf.htm
http://draco.acs.uci.edu/explorer/ping.html

I'd also suggest joining the Expedition Owners Mailing List. See
http://mail.xpog.com/mailman/listinfo/expedition.

Ed

With a System Lean report for both banks, I'd be thinking of a vacuum leak
at a location that is common to both sides of the intake manifold. Vacuum is
vacuum, and there are ports on one side or the other to provide vac to
various components. I think that if there is a vac leak on one side or the
other, you would see a lean report that has to do with that side, since you
have a lean report for both sides, I'd start my search closer to to the
throttle body, where a leak can lean the mixture to both sides.

The computer looks at the throttle plate (and a couple of sensors) to see
how much air is going in, then adjusts the injector timing to affect changes
to the air/fuel ratio (mixture). Your computer is telling you that the
injector timing has been set fully rich, but the mixture is still lean. One
cause of such a condition is a leak that is letting in air at a point
downstream from the throttle body that the computer does not
monitor/control, and the resulting air can not be made richer by the
addition of more fuel.

My guess is that you may be experiencing poor mileage too because the
computer is trying to add more gas to make the mixture richer, but the error
report you have says the effort is not working.

You could have failing O2 Sensors too. These look at the mixture leaving the
cylinders, and this feeds back to the computer which is looking at the
throttle plate and a host of other things to make decisions on injector
timing. But, there are specific codes for O2 sensor faliures, and I would
expect one of these codes ahead of a Too Lean (or Too Rich) report. The O2
Sensors are in the troubleshooting chart for your issue, but they fall well
below the vacuum leak.

If you have had any work done recently -- even an oil change -- then I'd be
looking for cracked hoses in the immediate vicinity of where the work would
be expected to be performed.

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