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2007 Forester
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Discussion Starter #1
I wondered if any of you had heard this before. I bought my Forester in 2010, with 43k miles on it and put in a K&N filter in it. I have about 118k now and have recharged the filter once.

Yesterday, I dropped my Forester off at the mechanic for new brakes, oil change and a new headlight. Car was running fine. I got it back and it was running a little rough with a very low idle and needed extra gas to get it moving. On the way home, I got gas and the check engine light lit up and cruise light flashed. I tried to bring it back to the mechanic, but it died and all the warning lights came on. I had to get it towed out there.

So the diagnosis was that there was a code for an obstruction in the throttle body with a fair amount of carbon build-up. They cleaned the air hoses, cleared the code and suggested upping it to mid-grade gas. He said he has had similar issues with K&N Filters in the past that fit this diagnosis.

Have you guys ever heard of this?
 

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2017 VW Golf SportWagen 5MT
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If he says he has seen this before, you can either believe him or not, choose.

I stopped using reusable filters on "normal" cars years ago, paper works great.

My guess as to why the car ran differently is the tech disconnected the battery and ECU got reset.

Stan
 

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2014 Forester XT CVT
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Did he perhaps do anything to the filter (like re-oil it)? ...... is the car running fine now after the cleaning....? Perhaps your MAF sensor is going caput?
 

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15 Forester XT CVT
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Just to give you an idea of what I think about K&N filters. I bought a Raptor it came with a K&N filter in it, I went to dealer paid 20 bucks for OEM filter, opened the air canister and pulled the K&N out. I threw it in the trash as fast as I could. Pulled the MAF and took some MAF Cleaner and sprayed it thoroughly. Reinstalled the MAF and the truck idles smooth as silk.

The oil in K&N filters clogs up the MAF sensor causing it to go crazy. On older carburetor or MAP cars I am sure it is fine but on Mass Airflow metered cars/ trucks, no way would I run K&N.
 

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2012 XT Touring 4EAT
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I wondered if any of you had heard this before. I bought my Forester in 2010, with 43k miles on it and put in a K&N filter in it. I have about 118k now and have recharged the filter once.
A K&N, you say? I had one of those once. It turned out to be really great for me. It was a crappy air filter, mind you, which caused me no end of headaches, CELs, screwed up the MAF with oil, etc. Finally, I took it out and tossed it on the shelf. OEMs are much less expensive and work just fine.

When I sold my first Forester, I noticed the K&N on the shelf and put it back in, mostly to get rid of it. The very first person who looked at the car noticed the K&N and was really impressed - he couldn't stop talking about it. I think it was what sealed the deal. K&Ns are A-OK in my book. Just don't use them as air filters.
 

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2014 Forester XT CVT
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Many people have used K&N air filters over the years without any problems (myself included....I have a K&N drop in panel filter in my SJ XT now that I dropped in soon after I bought it). The key is to not go overboard with the re-oiling of the filter after you clean it (you really don't need much) and to allow it sufficient time to dry and disperse into the filter element following re-oiling it. I also periodically clean my MAF sensor (every 10K miles or so) and have been doing so for many years. I have NEVER had any MAF related issues as a result of using the K&N drop in filter....

...and for those people that say it doesn't filter anything....I have had multiple used oil analysis done in the past on all my cars that also had a K&N...just had one done on my SJ XT.....none showed any increased silicone or insoluables (read dirt) than universal averages....

I just love how everyone quickly jumps to the conclusion that you will instantly foul up your MAF by even looking at one from afar... :rolleyes:
 

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2007 Forester
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Discussion Starter #10
It definitely helps to hear others have had issues. I probably could have cleaned the filter more. The thing that confused me is that once they cleared the code, the car has run great and it still had the K&N Filter in there at the time.

Could it have just been the MAF reset? Do you guys think running mid-grade gas will make any difference? He says due to the higher mileage, it will keep it running better for longer.
 

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2015 Impreza 5-Door Sport Auto (CVT)
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I only use genuine Subaru Oil Filters, but for air filters, Purolater PureOne filters work just fine. I think about $15 bucks or so.
 

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2017 Tacoma DCSB TRD OR 6MT!!!
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Two thoughts:
1. I've used K&N's on a Civic, Tacoma, Xterra, and now my Baja, and never had an issue. My cats went out at 130k in the Xterra, and my mechanic said it was likely because I used a K&N. I value his opinion, but respectfully disagreed with him. No other issues with any of the cars.
2. I had a dirty throttle body and MAF on my Baja (130k miles), and it was causing a rough/low idle. I paid my mechanic to clean the throttle body, and cleaned the MAF myself (carefully), and the issue improved. I hadn't even installed a K&N at that point.

I also agree with ForcedInduction: You have to clean and re-oil properly. I over-oiled it once and it caused by Taco to run a little rough for a while before working itself out.
 

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1998 Forester manual
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i bought one for my tacoma, stopped using it almost immediately. i was getting horrible fuel mileage with it. went back to paper and mileage went back up. called K&N, they did state that "yes some vehicles do NOT gain, some lose mileage"
 

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2007 Forester
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Discussion Starter #14
I'll have to remember to see if the replacement air filter has made any difference in terms of fuel mileage. I feel like it's running pretty well since they cleared the code.

I'm skeptical that running mid-grade gas will make any difference however. My wife has a Scion TC that burns oil and my mechanic said she should use premium to assist, but the Toyota dealer I went to said that was a myth.
 

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2006 Forester
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I have used K&Ns on two cars and one truck in the last 35 years with no problems. I was careful about cleaning/drying/oiling. I also had no significant seat of the pants gains in power [except I did think my SHO in the early 90s felt quicker in above the speed limit driving] and no discernible gain in MPG, but I liked having the longer service interval. In the late 80s I did SCCA and also my truck saw some rough duty, but since the late 90s my driving has been milquetoast ordinary.

I retired 6 years ago and my driving has gone from 22K+ per year to 7K per year. Thus I have no real use for the longer service interval and I now use paper filters in both vehicles [my wife drives fewer miles than I]! I have some observations about paper filters.

1] Subaru's claim 30K service while good aftermarket filters claim 12K. Check your filters with the see-through bright light test every 5K. You will probably want to change them at 10K-15K in normal driving, and every 5k if you drive off road, or on caliche. This includes the Subaru supposed "30K" filter.

2] Clean filters do provide a seat of the pants benefit over dirty ones! At under $10 for a good after market filter you can afford to keep them new and clean.

3] If you drive tons like I used to and you are careful with your bathing/drying/oiling technique, a K&N is a good choice. But don't buy it thinking you will either feel a power boost or measure a performance increase or mpg gains in ordinary driving. As part of a performance intake/exhaust/remap I have read it does make a difference so of course you should investigate if that is where you are going. And if you drive from the Red River to Ardmore OK after midnight averaging 107 MPH and don't get caught your K&N may be making a real rather than just a placebo effect difference!
 

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Those that off-road regularly often complain the K&N lets through too much dirt when driven in the dust, and if you don't drive in dirty conditions, paper will last a long time... so not that much benefit considering the cost and potential problems.

Any claims of performance (or anything else) when tried out on YOUR car makes the result subject to the classic problem of observer bias.
There is a reason drugs are tested with a placebo for much the same reason, because you may "notice" an improvement because you think there is or may be one, even subconsciously.

If you think there can be a mileage improvement, it is very easy to unknowingly modify your use of the skinny pedal, and a slightly more gentle application of that control can make a much larger impact to mpg's than any filter.

The same with performance differences..
Note many people are sure their car runs better after an oil change using the same oil type, or after a car wash.
It would be EXTREMELY (hen's teeth) rare for that to be the case unless your original oil was inappropriate or like asphalt or the car wash removed 50+ pounds of caked on mud... ;)
Yet, it isn't hard to find those that swear they can tell a difference.

Like @markinaustin noted, paper filters are pretty cheap, and replacing them when they are dirty is a very good and cost effective idea, and VERY likely that will be more often than 30K, regardless of what is written on the filter box.
An engine that can't inhale can't run well, but an engine that inhales "better" doesn't necessarily improve things.

An engine is essentially a pump, and pressure going both in and out need to be addressed to get any real benefit,
 
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