My 98 f150 has been a great truck but I keep getting lean codes for both banks. I have replaced fuel filter, it has a new MAF sensor, totaly rebuilt from heads up so there is no vacuum leaks, pcv valve is new, surge solinoid is new, idler motor works fine, all hoses are new but the gas cap is growling like my stomach does on my wifes samon patty night. I am thinking that the carbon canister is clogged and not letting the gas tank vent properly but could this cause the lean codes P0171 P0174

 

"Lean" codes are caused by either too much air or not enough fuel based on what the PCM expects.

Inadequate fuel tank venting will cause fuel starvation, especially under load. A fuel pressure gauge will answer this question.

 

Found one of the canisters clogged. Took it out and rerouted the lines to use just one cleared the codes and now I will see if the light comes on again.....