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Discussion Starter #1
So I finally got a test pipe made and installed a full 3" turbo-back exhaust (except for the 2.5" bend coming off the turbo on the Roush kit). Now that the cat is gone, I was able to install the AEM UEGO wide-band that I've had sitting around for awhile. The O2 sensor sits practically under the engine to guarantee the 36" from the exhaust port that the installation manual dictates. Before I installed the whole new exhaust, I tried putting the wide-band O2 in the bung that comes on the Roush 90 degree bend, but it was giving sporadic readings so I just removed it as that was maybe 6 inches from the exhaust port, not the required 36.

Now for the problem, or what I suspect is a problem. At idle, the AFR will fluctuate between 13.5 and 15.5. Occasionally, it will spike to 17+ then return back to fluctuating. The engine also feels like it's quite sure where to idle. It's still relatively smooth, or as smooth as it can be with all three VF motor mounts. While cruising, if I let off the throttle the AFR will usually peg lean like I would expect, but every so often it'll lean out, then become very rich (12-13) while engine braking. Sometimes it stays rich at idle, too.

I originally thought this may be a vacuum leak, but my vacuum is rock steady at idle.

While in boost, everything appears fine. I see 10.5-11.5 under full boost (~8 PSI).

Any ideas what could be causing this? Could a bad upstream O2 be a culprit?

Also, for those of you running with no cat and just a resonator and/or straight-through muffler, is it common to get some fairly loud popping coming from the exhaust while coasting around 2k RPM?

Thanks!
 

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All of the stuff you listed above is relatively normal. The idle fluctuations are completely normal as the ECU is always making adjustments to ensure it is near the goal of 14.7:1 AFR. When you let off the gas the engine essentially diesels which is why it goes lean. There's nothing wrong with it, except perhaps your O2 sensor is getting a little old. The max life of those is somewhere around 60k miles typically, but I'd change them a little more frequently to make sure you're getting the most out of your FI setup.
 

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Discussion Starter #3
I'm not as concerned about it pegging lean while off-throttle as much as I am it holding an AFR of 11-12 while off-throttle. Even if it's not harmful, it cannot possibly be good for fuel economy. [poke]
 

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hey that's all normal stuff even the occasional richness while off the throttle
 

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mine does the same stuff, and it always has.... so it sounds normal to me as well.
 

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Discussion Starter #7
Alright, cool! Thanks guys! [thumb]
 
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