O2 voltage drop

[slowblacktalon]

A buddy of mine is trying to tune his car and we can't figure something out. When we log it and tune for knock, everything is fine and dandy until he does a hard third gear pull and you can litterally look at his graphs on the datalogger and watch the O2 voltage drop from .86 to .27. this is a gradual drop of course, but it drops and we can't figure it out. It is not knocking when it drops either. Anybody heard of this happening before?

 

[psychlow]

His doesn't happen to be a '90, does it?

 

[4g63tsiawd]

why whats the deal with the 90 I'm getting my logger tomorrow so is there anything I need to know