Service Engine soon, DTC #p0171, p0174
Had an issuse with my 2001 navigator, running rough, throwing codes for lean on both banks codes P0171,P0174.
replaced the standard stuff, fuel filter, plugs, cleaned MAF.
Turns out it was the elbow for the PCV hose at the top of the engine underneath the plastic cover. It had cracked and caused a vaccum leak - apparently a VERY common problem. The elbow is a non serviceable item from Ford, you will need to replace the entire PCV hose, OR, you can go to C/T and get a PCV elbow packaged as a chrysler item, that will replace just the elbow portion.
Hope this helps someone else, it's probably an item you should look at evertime you replace the air filter.