What will cause a car to lean out?

[awdsm drft]

Car has leaned out on me for some reason. Didn't change my set up besides a front mount.

The car drove for a few miles under the richest condition possible when it broke a week ago so I believe my o2 sensor failed but the car still drives normal, its just the plugs are dangerously white.

Has anybody leaned out like this and what were the reasons?

 

[Quasimondo]

Mine leaned out from a faulty fuel pressure regulator. Check to make sure that the vacum lines between the intake, the solenoid, and the fpr aren't leaking and preventing your fpr from raising the pressure at WOT.