What to look for on a 2000 Taurus-Sable to correct P0171 P0174 Cleaned Mass air flow replaced coil 3 repaired vacuum leaks on valve covers?

look for vacuum leaks downstream from MAF if you find nothing the MAF may need replacing.

My 2000 Mercury Sable with a 24V DOHC 3 liter Duratec tripped the diagnostic trouble codes P0171 and P0174 (system too lean banks 1 and 2). I pulled off the 2 rubber elbow boots on the vacuum lines leading to the air intake covers and they were deteriorated (ripped apart as I removed them). Although they were 90 degree angle boots, I replaced them with 45 degree boots I found on a Taurus at the junk yard. The "check engine light" came on again on the 3rd trip. Ran my finger under the barely visible rubber elbow on the vacuum hose under the housing containing the MAF sensor (I presume the hose leads to the PCV valve) and felt a split in the rubber. To avoid the major hassle of taking off the air intake manifold, I wrapped that section of hose tightly with electrical tape. I erased the codes with my scanner and haven't had a problem since. My next steps would have been to check the EGR valve and sensor, DPFE sensor, PCV valve, and air intake manifold gasket.

It is also common to find loose clamps or splits in the snorkel between the MAF sensor and the throttle body, and it is also possible the small rubber gasket of the upper plenum to intake manifold is slightly leaking..... these will often only leak for a minute or so at a cold start then seal when warmed up, but already set the code when cold.