2008 Forester 2.5X Prem. 5spd MT
I pulled off my air cleaner to get to the spark plugs (just to check what the shop put in last november -- FR5AP-11 plugs apparently) and noticed some odd filter thing on the underside of the top part of the air cleaner, so downstream from the air filter (sits on top of it). I pried the thing off with a screwdriver just to relieve the restriction. Engine feels more responsive at lower rpm's and seems to haul harder at >2.5K RPM (very noticeable there), and after looking at the shop manual PDF's I noticed that's some kind of "HC adsorption filter" probably to collect hydrocarbons from the intake plenums so they can truly say it's a "PZEV" vehicle. Anyway I doubt I'll see any check engine lights out of this but I'm going to monitor it.