I currently own 10 Fords from 1965 to 2004. By far the 99 Explorer (127K) has given Ford a black eye. My check engine light is on again and the OBD II code is PO171 (too lean). I replaced O2 sensors on one side and the light appeared again a couple of weeks later. I cleaned the MAF, PVC valve and looked for vac leaks. Fuel filter was changed 12K ago. The engine is hard to start in the morning. Once started, it revs up and down for a few seconds and then idles. I have read online that it may be bad intake o-rings. Any similar experiences? Any help would be appreciated.
Look for leaks around the upper intake seals, damaged pcv hoses, vacuum tree at the back left, depending on the year, lower intake around the front or back corners. Also look for cracks, loose clamps on the intake air tube in between the ribs.