How to tell when your K&N Filter is dirty...

Easy- you get a check engine light and a P0171 code, but if you had a Miniscanner or a Canview, you could catch the condition much earlier by watching the O2 Fuel Trim..

That code actually means that the engine is running too rich (too much fuel, not enough air)..

This past winter has been fairly dry, and when the snow melted, we got a fair stretch of really dusty days, and I'd normally have swapped back to a regular Toyota filter by now, but we've had a rather wet spring and summer, which is where the oiled media excels, so I left the K&N in (just until the next oil change I kept telling myself- of course, I'm on synthetic oil and am running extended drain intervals, so...)

I did take it out at a few points along the way, and there wasn't much visible debris on the filter and it still looked red with oil, so I never gave it much thought and just put it back in.. Then I got a check engine light as I was accelerating onto the highway yesterday.. I quckly pulled over and pulled the code and it was the P0171 mentioned above.. The car didn't seem to run any different, and there were no other codes, so I went on my way and kept an eye on the various engine parameters and noticed that the O2 Fuel Trim was continually on the positive side instead of the usual +/- fluctuations, especially when accelerating, it would go as high as 25%, indicating a severe lack of air.. So I drop by the dealer and pick up a new filter and dropped it in and immediately, the fuel trim numbers came back to the normal..

I cleaned the K&N when I got home, but it really wasn't that dirty, but obviously it had trapped enough fine dust that it was doing a good job of restricting air flow, which does jive with that air filter report available on Bob is the Oil Guy where they show that the K&N has the highest initial air flow, but also tends to cause high restriction when only a moderate amount of dust is collected.. Based on this experience, I'm definitely going to be watching the Fuel Trim numbers more closely and inspecting and cleaning the filter more often, but if it happens again, I may just swear off the K&N altogether..

 

Bestas I can tell, I've put on about 9000 km since the last cleaning, but the point is that visual inspection of this kind of filter is simply not enough- as I said, I had checked it visually a number of times, including holding it up to the sun and it didn't look black/dirty and there wasn't any large debris on the filter, yet it had obviously oaught enough comtamination that the engine management system noticed the constriction it was causing, and thus, by watching the O2 Fuel Trim numbers, you can actually get an idea of whether your air filter (whether K&N or not) is perhaps becoming a bit restrictive..

As for MPG number, no really significant change, but if I had to say one directio or another, I'd say that the fuel economy has been better of late (which makes sense if the engine management was constantly reducing the amount of fuel metered to compensate for lack of oxygen..)

K&Ns do require re-oiling (the cleaning procedure involves spraying the whole thing with a cleaner (the instructions also say that any dish detergent is also usable), then rinsing it clean under the tap, which removes all the old oil and any collected dust, so quite obviously, you have to add fresh oil afterwards).. Adding a huge quantity of oil does not make the filter work any better (in fact it can make things worse as excess oil in the media can restrict air flow (anyone who has one of those bagless vacuums with washable filters knows just how much restriction a bit of moisture in the filter can cause if they've ever washed a filter and then got impatient waiting for it to dry and tried putting the filter back in when even slightly damp- the vacuum darne near keels over and dies because the surface tension of the moisture in the filter forms a nearly perfect barrier across all the little pores of the filter.. Addiditionally, too much oil is a very bad thing because small oil droplets can be pulled through with the air and contaminate the air flow sensor