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Have a K&N filter? When to clean and when not to (also, don't over oil!)


Anticept

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K&N air filters are used on a lot of aircraft, not just LSAs. However, old habits die hard as people clean them every chance they get. Here's the thing about K&N filters though: They work better when they are a little dirty due to the way the oil coating works. They should not be cleaned at every single interval!
 
I'll let K&N's own page show you when to clean: http://www.knfilters.com/cleaning-pics.htm. I wouldn't wait until it's THAT dirty, but don't panic just because it has some dirt specs.
 
It's also important to treat the filter properly. DO NOT USE COMPRESSED AIR DIRECTLY ON THE FLUTES. You will damage the filter's tiny filaments. If I have to, I'll blow the air at a flat angle parallel to the flutes to dislodge large debris. Usually the rinse gets everything though.
 
Here's K&N's instructions on cleaning and re-oiling the filter: http://www.knfilters.com/cleaning.htm
 
Finally: if the filter shrinks and is really hard to put on, use a heat gun or submerge the filter ring (NOT the filter element, just the end) in boiling water. It will soften the rubber a lot and expand a surprisingly large amount. I occasionally have to do this to get a filter on. They should just slide right on after doing this and when they cool, take the shape of the flange they are mounted on, so you shouldn't need to do this every single time.
 
Also: do NOT over oil the filter! This is especially true for any engines that have some sort of air flow or mass air sensor. K&N oil is rumored to damage those sensors and it's still an ongoing controversy. Still, K&N specifically says do NOT over oil the filters! (@ http://www.knfilters.com/filter_facts.htm)
 

Over-oiling
When servicing a K&N filter, take care not to over-oil the element. Besides impeding air flow, excess oil can migrate into the intake system where it can coat electronic sensors, which some OEM's claim may hinder the sensors' operation and result in a repair that will not be covered under warranty. Although K&N disagrees with such claims, as explained in more detail on the Mass Air Flow Sensor page of web site, in order to avoid a dispute with an OEM over the denial of a warranty claim, we suggest that you be careful not to over-oil your K&N air filter. Never saturate the filter. If oil drips from the filter, wash it and start over. Use only K&N oil. For example, an E-1500 filter has 92.4-inches of surface area requiring 1.707 fluid ounces of oil. Follow oiling instructions included with your filter or refer to the K&N Air Filter Cleaning Instructions.

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Interestingly enough, paper filters are effectively non-existent in standard airworthiness small aircraft in the united states. AD 84-26-02. Note there aren't any paper air filters that are approved anymore by the FAA (as far as I'm aware).

 

If you're using a paper air filter, I strongly advise replacing it. The cause of that AD were multiple instances of moisture softening the paper and subsequent ingestion and engine stoppage.

 

Besides, it takes a few hundred hours before these filters are dirty enough to warrant cleaning. Obviously those in dusty places will need to more often, but it's still pretty rare. Given all that, K&N filters are better than paper.

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Corey, you should read the AD. Paper filters are allowed. The AD comes from poor maintenance practices. People were not following manufacturers recommendations and changing the filters like they should, so the FAA stepped in and made a AD. Paper filters must be replaced every 500 hours per the AD.

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Corey, you should read the AD. Paper filters are allowed. The AD comes from poor maintenance practices. People were not following manufacturers recommendations and changing the filters like they should, so the FAA stepped in and made a AD. Paper filters must be replaced every 500 hours per the AD.

I've never seen a paper induction filter installed anywhere. I had asked A&P school instructors about it a couple years ago and they had said there aren't any anymore, it's all different media now because of the ingestion issue and pointed me at that AD.

 

Granted that's not an official enough source, but I did a couple relatively quick searches and didn't find any on aircraft spruce that are approved.

 

It just seems too convenient. Maybe instead I will say "effectively non-existent" vs "not allowed".

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The directions on my spray can of K&N oil says to spray every pleat at a distance of 3" (don't have the can near by so I'm not 100% sure about the 3" distance but am sure about spraying each pleat). This results in a LOT of oil being applied.  It seems to be way too much to me.  Is it OK to just give the filter one or two slow passes at a foot or so distance to give it a good even covering?

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