I can't speak for the original poster. This has been my experience. Have a '99 with 125k miles that continues to generate P0171 codes. Cleaned the MAF - no change. Replaced the A/F Sensor - Ran better, but continues to give the P0171.
My OD is working despite the P0171. When I go wide open throttle or close to WOT, the engine completely chokes up. Loses power, barely accelerates. Until I cross that threshold, the engine seems strong.
My father, who is a retired Ford mechanic, is convinced it is a vacuum leak. I have not had any luck finding the vacuum leak yet. I'm going to take a look for that hose on the back of the air box next. After that I'll look for the vacuum leak a few more times before I start replacing more parts.
