Code 171 error ford expedition 1998 - 2007 Chrysler PT Cruiser Touring Hatchback

There are a multitude of things that could result in a PO171 trouble code.
It might be the infamous PCV elbow problem.. Around 100K miles 5.4's are bad to rot out the PCV elbow on the back of the throttle body, this will cause bad idling problems and will throw lean codes when it gets really bad.
Find the PCV valve in the passengers side valve cover and trace the tube around to the back of the throttle body. With the engine running feel around the rubber elbow and see if you have a vacuum leak.

Here are some other things to try:
1. call your local dealer and ask them what it could be
2. Replace the front two Oxygen sensors.
3. Clean MAF sensor (especially if you've installed a K/N filter - could coat the wires with oil that collects dust).
4. Check/change air filter
5. Change the fuel filter (under drivers side frame)
6. Check fuel pressure

Hope this helps, Good luck.