Blow off valve

a.k.a. "BOV"

Valve which relieves charge pressure by venting air to atmosphere.

Prevents turbo surge, but may affect LTFTs.

How? BOVs can release measured air from the intake system, and when they do, they make the ECU "think" there is more air in the combustion chamber than there really is. This will cause the car to run rich during closed loop operation, and to compensate, LTFTs will go down. However the car may still run rich (or lean) during closed loop operation, since the LTFTs only reflect recent condions, and if the BOVs are venting more (or less) often than they were recently, the LTFT will be wrong. To make matters worse, the car can also run rich open loop, again depending on how much air was lost while the BOVs were venting.

This fudge factor (both in partial and wide open throttle operation) is almost impossible to properly correct for, since it varies wildly depending on driving condtions and driver behavior.