The check engine light comes on at 50 MPH

[FordExplorer87]

:exporange my dad's 00 XLS that has 87,500.00 miles and the check engine light come on at 50 mph and i put on the Scan tool and i get 2 DTC PO171 and PO174 or System TO Lean on bank 1 and 2 so i started cheap with a new gas cap and cleared the code and 4 days later the check engine light comes back on onDemand it say to much purressure or vacum leak after the MAF senor or new HO2 senors :exporange

 

[Glacier991]

Difficult to understand your post in the 2nd part. What were the codes you got the 2nd time?

 

[jp450]

Those codes indicate that air is getting in past the MAF. Normaly its the intake gaskets. Check for loose or broken vac lines, missing caps etc... If they all look good then replace the intake gaskets. They cost about $30 from the dealer and take someone with reasonable ability about 1 to 1.5 hours to do. I would recomend getting a manual with the diagrams to assist you.