So I've got this K&N 57i series intake marketed as a match for the US/Canada 2005 Corolla, but I've been having some difficulties with it.

For anyone who hasn't seen this intake kit, it is kind of neat. The only parts that are installed are an AL tube for the MAF and the filter adapter+filter; which makes this a minimally invasive way to put a little more kick in the 1zz-fe. It also comes with a CAI hose which you can snake down and zip-tie to the transaxle mount, allowing delivery of more air to the intake without the drawbacks of putting a filter down there. I was attracted to this intake because I drive to see my family (about 170mi on a two lane highway) a few times a month and have had real difficulty passing people. At around 5600RPM in 3rd with the stock box I could rarely make a pass possible, but with the 57i kit, I really do get an extra few seconds to complete the pass before colliding into an oncoming car. I don't think the kit does anything significant for the car under 4000RPM, but I only wanted it for the high range anyway.

So here is the trouble, according to K&N:

http://www.knfilters.com/check_engine_light.htm

They claim the kits specifically for any given vehicle will not case any CEL trouble, in kind of wishy-washy terms. However, I have been throwing the p0171 (Lean, Bank 1) code almost daily for about 4-5 days after I installed the thing. I installed my new Orange silicon Intake Manifold Gasket at the same time (went through about a can of carb cleaner to clear the intake manifold and throttle body).

When the code first appeared, I replaced my throttle body gasket, spark plugs, PCV, and MAF sensor. Then I piston soaked with MMO for 30hrs, evacuated, then seafoamed 6 fl oz via brake booster. After that I filled my tank w/added Techron concentrate (BTW I work at O'Reilly so all of this was not nearly as expensive as you'd imagine; the discounts are pretty unbelievable).

The car runs amazingly well, but it did not take more than 20mi for the CEL to pop back up.

This is the point I'm at. I just finished reinstalling my stock air box to attempt to identify whether the intake is the sole cause of my trouble. In the next few days I will see if that CEL comes back. The only things left I can think may be causing the p0171 are: defective intake manifold (the thing is a POS made of plastic, it has been out of the vehicle a few times, and I maybe shouldn't have used carb cleaner on it; could be warped or cracked), a vacuum leak somewhere (I used teflon on the PCV, no leaks there; but I haven't thoroughly investigated the system for leaks yet), or a failing o2 sensor (failing sensor will sometimes throw codes, but i doubt the sensor coincidentally decided to fail when I installed the intake).

I didn't buy this intake too long ago and am not entirely thrilled with the idea of trying to return it, particularly since it is helpful on the highway (I had tried a drop-in filter first but it caused odd idling issues). Aside from the CEL it idles perfectly quite (quite as a 1zz can be) and is more peppy than usual while maintaining acceptable miles per gallon (about 33 on average, but I have uneconomic tires using low PSI which leech about 5mpg in exchange for fantastic handling). But the bottom line is the car cannot run lean, not for very long anyway. I'm not interested in destroying my engine in that manner.

I guess the point of all of this is to see what you guys have to say about intakes for the 9th gen corollas w/DBW: what works, what doesn't. If you can think of any possible solutions to this problem I am having I'd love to hear it as well.