2004 mercury monterey p0174 bank two running too lean

I have an 04 mercury monterey with the cel on giving the codes p0174 (bank two running too lean) however I can't figure out what is causing this! First I cleaned the maf air flow sensor that didnt fix it!! Then checked all vacuum hoses and can't find any leaks! Then I changes the pcv valve! That wasn't it.... Changed out the upper O2 sensor that didnt do it! Last but not least changed the plenum intake mainifold gaskets! Mind you I took it to master muffler to perform a diagnostic and smoke rest and they were the ones who recommended the plenum gaskets be changed!! If anyone has any advice Please help me out! Tag has been expired for two months now and it wont pass emissions!