More Trouble Code Questions - P0171 & P0174

Britt, no, a gas cap won't help with those codes.

The SOHC 4.0 is notorious for having leaking intake gaskets, which give a lean condition and set those codes. I'd try snugging up the intake bolts first, and see if that makes any difference. Also make sure there are no air leaks at the air tube between the MAF and throttle body. If the PCV hose attaches to the air tube, make sure it's tight and leak-free.

Sometimes cleaning the MAF sensor wires will help. I use alcohol in a spray bottle, but you can also use brake cleaner if you're careful. The wires are very delicate, and you don't want to break one. A search should turn up all sorts of info on this.

If none of this helps, then I'd consider replacing the intake gaskets.
Good luck.