ok guys, got a good one for you all, any help would be greatly appreciated. Ok, start at the basics, 97 ranger 4.0 5 speed 2wd with about 115k miles.
check engine light kicked on, ran a code reader on it and got 0171 and 0174, running lean on both banks, ok, so in order to "fix" it (or i hoped anyways) I have changed spark plugs, fuel filter, rear o2 sensor checked for vacuum leaks, checked to see if pressure regulator was bad, changed mass air flow sensor, after all changes check engine light kicks right back on within 10-15 miles, this one even has the local shops around me confused, i dont know if im the only 1 to have this trouble, or if its fairly common problem, if anyone else has experienced this OR knows a possible cause im open for ideas, thanks

Here are the common problems:
leak between the MAF and the throttle body;
leak in the PCV system, especially the hose elbow;
leaking intake manifold gaskets;
any other vacuum leak;
"lazy" oxygen sensors (the front ones).

The rear sensor has nothing to do with these two codes.

your money would be better spent having a shop diagnose them. IF a local shop cannot tell you what the problem is with those two codes for about an hours worth of shop time, then you need to find a better technician!

I would focus my energy on the list of possible causes that Bob posted, the resolution of the codes is somewhere in there.

Regarding the '97 4.0L, another thing to check is the fuel supply manifold (a/k/a/ fuel rail) gasket per the Ford TSB.

Article No. 98-6-8

LIGHT TRUCK: 1997 AEROSTAR, RANGER

This TSB article is being republished in its entirety to correct the part number listed, revise the Service Procedure, and add Aerostar.

ISSUE:
A rough idle, stalling at an idle and/or Diagnostic Trouble Codes (DTCs) P0171, P0174, P1131, and/or P1151 may be stored in memory on some vehicles with a 4.0L engine. This may be caused by the gasket located between the fuel rail and lower intake manifold losing its compressive loading and becoming drawn into the intake manifold runners resulting in a vacuum leak.

ACTION:
Replace the fuel rail-to-intake manifold paper gasket with a revised gasket. Refer to the following Service Procedure and the 1997 Ranger or Aerostar Service Manual for removal and replacement procedures...

PART NUMBER / PART NAME
F8PZ-9E436-AA / Fuel Rail-To-Intake Manifold Gasket

hey thanks for the ideas of where to look, also i woulda never thought to check the fuel rail gasket, gives me a good couple of places to start looking on it, thanks for the help, ill update after i check them areas to let you guys know how it goes

well, been busy for the last... while, anyways, got around to checkin my truck out, decided to re-check the maf and found it to be defective, even tho it was brand new, so got a replacement and cleared the codes, since then its been clean, thanks for the pointers tho, it was still much appreciated