O2 sensor gone mad? (graph attached)
 

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I assume that you're using the stock O2 sensors instead of wide band sensors?

If so, the O2 sensors are only really linear in AFR near stoichiometric. Like 13.5:1 to about 15.5:1. Outside of that range, the reading goes non-linear and is pretty much useless. So what we are seeing is that you are in cruise and the closed loop AFR system is running it's algorithm to keep the car at about stoich. To do this, it has to swing above and below that point constantly (because you're making little changes to the throttle, there are little changes in load, ect..). It does look a little wild to me, but it may be normal for our cars. You would need a wide band to see if your car is actually running rich or lean at varying loads.

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Is this the first time you've taken the new exhaust to redline? Maybe you just need to get used to the sound?

A blown out cat is a little loud, you still have a muffler, an exhaust leak is very loud. I've blown through a couple of cats with my Rx-7 and the car does get louder, but it's gradual and still not exhaust leak loud.

I'm sure you've done it already, but recheck the exhaust system for leaks and retorque your bolts. Maybe you need new gaskets or have a warped flange that only shows up when the pipes are hot. Chances are it's not the cat.

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Good, I'm glad it worked out. I understand what you are saying about driving your car. There aren't many attention grabbing cars in China. Lots of Buicks though.

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