1990 F-250 5.8 idle problems

Cannot get it to idle smooth. Its a 5.8 1990 f-250. Cannot find vacumm leak. KOER test said code 41. Leads to o2 sensor. Changed it still same. Changed Pantiurm ( or how ever spelled) gasket due to changing vaulve cover gaskets. I did find if I take the vacumm lines off the carbon canister and hook the toghter it runs like a new one. Could it be the canister of a vacumm leak somewhere elese. The truck only has 6800 mile on it. HELP!!!!!!!!!!!!!!!!!!!

I had the same thing wrong with my 94 it was the votage regulater