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2018 subaru wrx premium
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Discussion Starter #1
Hey guys, I already asked my tuner, waiting to hear back, but what do you think may be causing lean conditions under light throttle? At cruising or very light acceleration under 9% throttle I've noticed my car getting very lean af, 16 up to 18 sometimes.
 

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Using what to measure?
 

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Although I do understand that the FA20 uses a true 3-wire WBO2 sensor, I'm not certain the precision of that sensor (it does not need to be precise the way that it is used).

Something that you should consider doing is looking at the parameter "Final Fueling Base."

If under light load, the ECU should still be operating within Closed-Loop Fueling logic, so comparing against FFB will tell you if there is a discrepancy between actual and expected.
 

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It might be called Comm Fuel Final on Cobb

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Discussion Starter #7
Okay I found it. So basically I want the two to stay pretty close to one another right?
 

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If the onboard O2 sensor can be reliably used to monitor AFR under load, yes.

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Discussion Starter #9
Okay, so what I've noticed.. at idle fuel final says 14.64 and af stays pretty much there but bumps up to 15.2 occasionally.
While cruising it generally stays where fuel final says, but once and a while it does lean out to around 16, don't usually see it hit 18, but it has, while fuel final says I should be around the 14.6 14.8 area.. And again, it's only under very light throttle.
 

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Then don't worry about it.

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NP.

This is normal for MAF-metered ICEs. The MAF signal lags the pressure wave into the cylinders and so the ECU compensates by increasing injector duty cycle just a bit on accelerator input. It's never perfect though. You may see the AFR swing slightly lean or rich for about half a second on accelerator input.

Similar in concept to accelerator pumps in carbs. This is called "Tip-in Enrichment."
 
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