I just bought a 99 f150 and the check engine light came on with code p0171,system too lean bank 1. It runs and drives fine but has a strong smell of gas when I shut it off. Any ideas?

 

I cleaned the maf sensor and noticed oil residue in the air intake tube after the maf. I have a stock air cleaner also

 

You have an important clue with the smell of gas.
Find the source and you likely will clear the code issue.
The code says lean but the basis for it is the fuel table has shifted to it's lean side, out of limits that sets the code.
The cause is likely the excess fuel you smell from a leaking injector or even a fuel regulator diaphragm leaking fuel through the vacuum hose.
Good luck.

 

I thought maybe the strong fuel smell was from the PCM trying to compensate for the lean condition by adding more fuel.

 

No, that would be an out of control action.
It does not work that way.
The system can only correct to a range of about + or - 20% on the software tables it runs on, not unlimited changes.
Good luck.