lean issue ??

hey guys i recently got a 86 turbo z, it had some stuff done to it and i've been sorting out the previous owners mistakes as i go along. it has a straight 3 inch exhaust from down pipe back cold air intake on it. i have a aem wideband 02 in it roughly 1 1/2 to 3 ft from the original 02 sensor my issue is when im cruising or idle it runs lean between 15.3 to 15.8, wot is between 11.9 and 12.4 i pulled plugs they are grayish with a hint of brown barely any but can notice it cap and rotor are new as well on it, also has gutted upper-intake plenum on it running about 8-10 psi. could it possibly be bad sending unit, or wide band location, or possibly faulty readings from the wide band 02, exhaust leaks, or factory 02 on its way out ? any ideas on where to start would be much appricated