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Old 10-23-2012, 02:22 AM   #11
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Originally Posted by cowboy_Rob View Post
Just swapped my intake to a kstech mega maf and while I'm working on scaling it I noticed that at idle when my stock o2 is reading 14.7 on the logger my AEM uego reads 15.2. Which one is right or is there any explanation for them not matching?
At idle the ECU is generally going to aim for 14.7 or a hair richer. If you want to know exactly what it's shooting for, you can log the closed-loop fueling target AFR and/or the "final fueling base" parameter. The latter includes compensations that can sometimes be surprising. (Those are the names in RomRaider's logger; the names in Cobb's logger might be different, I wouldn't know.)

So one theory is that your stock O2 sensor is a bit messed up and it reads 14.7 when the actual AFR is 15.2. The ECU hasn't recognized the problem, so it believes the sensor. If it gets bad enough you'll get a Check Engine light.

The other theory, of course, is that your stock O2 is working perfectly and your AEM is off by 0.5.

You might be able to use the rear O2 sensor to break the tie. I have never really paid attention to my rear O2 sensor readings, so I don't know if this will work, but it seems plausible...

Keep in mind that the rear sensor is not very accurate unless you're close to 14.7. But if it says 14.7 I'd believe whichever wideband also says 14.7. Try logging the stock O2 sensor, the AEM and the rear O2 sensor, and see what happens.

If you're cruising at a steady speed, the ECU will start oscillating the AFR a little bit rich and a little bit lean. All three sensors should bounce above and below 14.7 together. If only two of them actually do, then you know which sensor is hosed.

Also, I moved my stock sensor into my DP bellmouth and nothing changed, so I wouldn't worry about where yours is located. It reads within about 3% of my PLX wideband, except that it won't read richer than 11.1 or 11.2.
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