what does an exhaust leak do to your O2's & BLM's?

[325DR]

I have an exhuast leak between the exhaust manifold and the turbo (at the 3-bolt flange) and was wondering if that would make the O2's read lean, or if it would make it read rich. Also, would that make the BLMs higher or lower? Thanks!

 

[Mile Hi GN]

A pre-O2 sensor exhaust leak will make the O2's lean, and the BLM's head for the sky. A post O2 sensor exhaust leak won't effect O2's or BLM's, opening the dump, etc.

The turbo flange is located just downstream of the O2 sensor so I don't think it would impact it's readings.

Now...the leaks effect on spool-up is a different matter