Gas cap is good start. One thing i observed when I was having these recurring codes, was actually a few things. 1. The hoses attaching to the canister, at least one of them was a flared fittinf meaning 3/8th on one side 7/16th on the other I got mine fixed using a length of each size and a coupler and hose clamps. The lengths of hose I got were very short lengths maybe 6" at most per segment, AutoZone gave me them for scrap hose but I had to pay for the couplers, and clamps. One more area to check is off the PCV valve there is a hose that runs into a plastic tube across the manifold and connects with another piece of 90 degree elbow in the back of the engine drivers side between the firewall and engine, that elbow was split. Check both pieces of vacuum hoses both on the engine and pcv valve side. Really check the hoses good, mine were dryrotted and split at the fittings. Might be a good thing to check prior to a long road trip, saves some embarrassment while out on the road. Even with the codes, my 4.7 fine, just a nusciance and distraction and possibly a slight fuel mileage drop but fuel drop is negligible. GoodLuck!!!!