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First post here, thought I would share something I've been trying to track down for awhile. I purchased a 2009 NA Forester almost 2 years ago with 145k. It has 155k miles on it now. Soon after purchasing, I replaced the head gaskets, diff fluids, ATF fluid, air filter, spark plugs, wires, and ignition coil as part of general maintenance. I have a scangauge connected and have had a persistent negative long term fuel trim at around -10 to -12%. I threw a mass air flow sensor and an upstream oxygen sensor at it but that didn't change the negative fuel trim. I replaced the PCV valve and cleaned the EGR, no changes. I then read that others were removing the charcoal air inlet filter (part of the PZEV badging) to improve performance. Removing the charcoal filter seems to have solved my negative fuel trim problem. I am still mostly negative but it doesn't drop below -5%. I assume that the computer doesn't need to pull as much fuel now that I'm getting more air flow. Not sure on the exact reason since the MAF is located downstream of the charcoal air filter and should be adjusting fuel based on air flow. Anyway, I didn't come across any other problems related to a clogged charcoal air inlet filter correlating to negative fuel trims. This may be a solution for others.
 

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2015 Forester "Premium", 2.0 NA
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Very interesting! I would love to hear more about this from other people. I don't think our non-USDM Forester has the charcoal air inlet filter, but i'll have to double check next time im changing the air filter
 
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