K&N Air Filters



I just got a brand new car, 08 fusion se. I have used K&N filters in the past with good increases in mpg. I have heard recently that some people are saying K&N filters are bad for engines because they let more contaminants through. Does anyone out there know if this is true? Or has anyone hard of an engine failure or issue related to a K&N? I want to help my mileage, but obviously not at the potential risk of hurting the engine long term.


They do indeed let more dirt through as well as increasing oil and grime buildup on the Mass Air Flow sensor. A K&N filter is a wire mesh with an oiled cotton-like gauze between it. While it will prevent large pieces of dirt from getting through, it doesn’t do so well with smaller particles. They make sense on performance engines that need to squeeze out extra horsepower, but if I was going to keep a car for a long time (150,000 miles+) I’d pass on it. A traditional paper filter or synthetic media air filter would be best in the long run. If you insist on having a K&N I would suggest changing your oil more often.


Here is a prior thread about this



It’s been pretty well demonstrated that K&N’s filter somewhat less well than a paper filter, but it’s less clear if that’s a serious problem-- if anything, it’s a matter of miles being skimmed off the end of an engine’s life and it would be very difficult to quantify what effect, if any, the air filter would have had. On a new car, I wouldn’t do it because even though K&N claims they don’t void the warranty, if you have engine problems and they find out you’ve been using a K&N, they will defintely try to claim that it was the issue and that it’s your fault.

Also, it is probably less likely that you will find any significant improvement on a newer car. Older cars had more MPG’s up for grabs, since design was more of a hodge-podge than it is now and fuel economy was only a minor concern. Your Focus was designed front to back with fuel economy in mind, and the engineers have already compensated for the flow restriction caused by the paper filter-- chances are the engine won’t need the extra air the K&N will flow through.


Thanks for the link and the advice. I didn’t think of the warranty issues GreasyJack, good point there. I’ll end up leaving the thing alone. My dealership gives me free oil changes for life, so I guess I can handle the cost of replacing the air filter, hehe. Thanks again guys.


Have used K&N on new cars for about 10 years. Sister uses one on her new Lexus with no problems. Her mechanic told her it would not void the warranty but he wouldn’t use it. He couldn’t give a reason, though. She did anyway and for about a year now she has no probs. Fram makes the same thing called, Air Hog. I like the quicker throttle response they give. Have used them on Supra, Malibu, Aleros, Ranger truck. Very happy. None of these are obviously performance vehicles except for the Supra, maybe.


For every gallon of gas your car consumes…it will consume 10,000 gallons of air. I’d want the most filtered air I could get.




I use an oil-gauze filter on my Mustang, mainly because when you have forced induction air flow becomes more important. Right out of the box they will not filter as well as a paper filter, but one some dirt accumulates on it will filter about the same. As for K&N’s ruining MAF sensors; 90% of the time these failures happen right after the user has attempted to clean the filter and put too much oil on it. When you re-oil the filter correctly there is no issue.



A K&N filter does NOT become the equal of a paper filter with use, not even close. Random strands of cotten with oil on them cannot accomplish what a paper filter can.

I just pulled a K&N out of a BMW a few months ago that had wreaked havoc because over time because it developed pin holes, it didn’t filter well, and it coated the intake with oil residue. They have their place, but they are not effective filters compared to paper, and miles away from synthetic media.


Very little gain in power, maybe even a little loss, no gain in mileage. While there is some evidence of possible engine damage, I also believe that is also very small and unlikely, except for cars with MAFs Just a little too much oil on the filter and by by MAF.

Check out:



In short, don't bother

From VW

drop-in/direct replacement filter is generally less problematic due to the retention of the factory airbox and intake tubing, but problems may arise if the filter panel does not fit as well as the OEM panel. Aftermarket drop-in replacement filters often have fewer pleats or folds, reducing the overall surface area available for filtration. While the media itself flows more when clean, the area through which the air may
travel is reduced, negating much of the claimed advantage. Additionally, the cotton gauze’s filtration properties are not as good as paper or foam, especially when new/clean. It has been determined that most engine wear is caused by particulates between 10 and 20 microns in size. Many popular cotton gauze filters are unable to effectively filter particles below approximately 22 microns. The use of either type of replacement filter may also increase turbulence of the intake charge, causing fluctuating airflow readings from the MAF sensor. This may set a DTC and cause the check engine light to illuminate.


If you drive in a manner that results in maximum fuel economy (light foot on the gas, specifically), you’ll rarely if ever experience the theoretical improved air flow a K&N filter might have.


The improvement in mileage from K&N is marginal at the upper limit of operating rpms and power output. In long term use they appear to save money if you compare regular cleaning to regular replacment. Like many higher priced maintenance products, those who buy them are often the people who habitually take better care of their cars and benefit from their dilligence even if some of their efforts are over-kill or marginal in effect.


What I don’t get is, you pay $60.00-$70.00 for an air filter that doesn’t protect the engine as well as an OEM type air filter, doesn’t provide any benifit unless you run the vehicle at wide open throttle, and you then have to purchase recharge kit for $20.00 and go through the hassle of cleaning and re-oiling the filter? With the possiblity of damaging a MAF sensor?

I don’t see the economics!



you can purchase the oil bath,at any local cycle shop (cheap) and then save the 20.00 for the kit.and the bottle from the cycle shop will last longer than the car.you will get many re-charges from the bottle purchased. just a suggestion.

The re-charge only takes a few minutes.


why is the oil kit necessary? what exactly does it do?