P0171 on a 99 Expedition

Anyone have any ideas as to the cause of this code? It came on after I replaced the passenger side valve cover gasket.
Thanks

Sounds like you may have a vacuum leak that is only affecting the passenger side. Look to see if you left anything unplugged.

Also too your MAF sensor could be dirty or defective. I would first check for vacuum leaks though like mentioned above. You could take some carb cleaner or starter fluid and spray around your hoses with the engine running and if you notice any difference in the idle that is where your leak is. Write back when you find something out

Sprayed with carb cleaner and starter fluid and I did not get any hits. I also sprayed the MAF with carb cleaner and disc brake cleaner. So far light is still on but I have only driven about 50 miles since cleaning the MAF, so the light is still on. Any other ideas? The truck runs good as is. I notice just a increase in gas usage. The idle is steady and I don't have any hesitation. Help

I found a broken vacuum hose coming from the pcv to the back of the intake. The plastic hose connector is a 3/8" and 3/4" adaptor. The dealer had a a new upgraded replacement. Thanks for the help.