[fire239]

Ok...My exhaust has a crack in it where the tubes merge into the collector. I know this is a common problem, but exactly what affect does this have on performance, mileage, etc. I'm thinking that exhaust leaking just upstream of the O2 sensor isn't a good thing though. It is a large crack too, from one side to the other when looking from the ground up to the exhaust.

[Sweeney]

If, due to scavenging, air is being drawn in, the O2 sensor will signal a lean condition resulting in a richened mixture. This may make a little more power but will also increase fuel consumption. If it is blowing exhaust out, it may have a slightly detrimental effect on any scavenging that may be occurring.

[dyna]

And if bad enough load the converter with xtra fuel from rich running leading to a early death.