first off i looke around and could not find something simular to this problem.,,

when im driving at say 60MPH and have my foot steady on the pedal my car will go very lean and throw a o2 code. i know its lean because i have thrown in a AEM wideband just to see what its doing and its going lean where the wideband dont recognise it.. ive changed plugs, wires, fuel filter, air filter, next would be fuel pump (walbro 255) if i need to

 

Try Iat sensor my z6 used to throw a O2 code changed IAT and went away