What a new air filter does

This weekend I put a new air filter in my 2002 Kia Sedona mini van. The one I replaced was pretty dirty and had least 12,000+ miles on it if not more. My dad said I should see at least a 3-4 miles per gallon better gas milage due to the new clean filter, I think that is kinda high for something as simple as a new air filter. Who is right?

You are. A dirty air filter does not hurt mpgs on a modern fuel injected car, unless it’s plugged to the extent that it interfered with normal engine operation. The computer keep the right amount of fuel injected for the amount of air going in, which is controlled by the throttle, not the air filter. It could cause a slight drop in full throttle acceleration, but that’s not the question here.

The computer will compensate to a point, but do not interpret that as suggesting that you can keep going 'til the engine starts coughing. In addition to affecting performance when it becomes too plugged, the crap that it’s plugged up with can move through it and contaminate the Mass Airflow Sensor (MAF). That can mess up the engine operation overall, and can also affect mileage.

Filters an fluids should be changed on schedule, in accordance with the recommendations of your owners’ manual, as should all other maintenance be done.

Before fuel injection a dirty air filter would be very costly. On a large V-8 engine driven at freeway speeds 3 to 4 mpg would be a reasonable estimate of the cost in mileage. Current EFI systems mostly lose power at highway speed.

Knox is correct…

12K miles on most air filters in current cars still have many miles left in them unless you spend most of your time driving on dirt roads. 30K is the norm for replacement intervals. The quick lube places show customers “dirty” air filters and get them to buy replacements way before needed. An air filter can look a bit dirty and still do a good job. Replace every 30K miles is good policy.

Since you replaced at 12K miles you will see zero difference in your mpg.

A Shop-Vac can greatly extend the life of an air-filter…

You may gain a little but it won’t be anywhere near 3 or 4 MPG. As to replacement intervals, that depends on the environment you live in.

Here in OK it’s quite easy to replace an air filter and then have to replace it again 5k miles later, although many people do not do this.

Some years I have to clean out my home central A/C unit condensing unit twice a year; once in early spring and again in mid summer. That kind of contamination is carried over to automotive air filters also and just like those filters, many home units are also neglected.

“A Shop-Vac can greatly extend the life of an air-filter…”

I would not use that 1960’s trick. While it will sort of work, it is likely to be less efficient and effective. That is it will not allow the car to breath as easily and it will not keep the incoming air quite as clean. Are filters are generally cheap, I would go with a new filter. I believe it is going to be cheaper in the long run.

I don’t know about cleaning an air filter with a vacuum, but this site says it’s harmful to use compressed air:



Shop Vac may get the dirt on the surface of the air-filter…But won’t get any imbedded dirt in the paper element itself. It may last a little while longer.

On my wifes Lexus who’s air-filter is a pain to replace…It’s NOT worth going to the effort to remove the filter and NOT just replace it.

As for gas mileage increase…As most people have said…unlikely it’s going to effect gas mileage much. I do like keeping a clean air-filter. Why take the chance of dirt getting inside the engine. For every gallon of gas your engine uses…it’ll use 10,000 gallons of air. That’s a lot of air passing over a dirty filter.

I am beginning to form the opinion that an automotive air filter that appears to be almost as clean appearing as a new filter can be used indefinitely up to 50,000 miles for now, depending on the dust level in the air where you drive. I use compressed air to clean out the valleys on the intake side between the pleats with a final blast of air directed from the inside to the outside to clear out dust that may be loosely imbedded in the filter media.

The sites posted do not provide details describing their method of cleaning air filters. I get the impression from the Bolivian links provided that they are a pretty good effort for the most part but have some snake oil selling aspects.

Agreed, a slightly dirty air filter does not affect gas mileage with fuel injection. The same is true with my diesel car but my air filters do not appear to get very dirty according to the bright light inspection method.

I confess, guys, I’m a wee bit concerned that everyone here seems to be suggesting that an air filter can be used virtually indefinitely, and perhaps doesn’t need to be even cleaned until it begins to affect operation. Are we sending a poor signal here?

" I’m a wee bit concerned that everyone here seems to be suggesting that an air filter can be used virtually indefinitely, and perhaps doesn’t need to be even cleaned until it begins to affect operation. Are we sending a poor signal here?" (the same mountainbike)

 I don't think many of us are suggesting that, but I don't think we have all been as careful with our comments as we should have been.   It is easy for the reader to read " a slightly dirty air filter does not affect gas mileage with fuel injection." with - there is no reason to change the filter until it does interfere with the operation of the car.  

 Air filters are there to protect the engine and should be replaced no less often than recommended by the car's manufacturer.

I don’t bother to clean an air filter. At 25K miles I buy a replacement at the parts store and put it in when convenient. I don’t go beyond the 30K interval that is spec for all my cars. Replacement filters are about $10.

The '03 Civic has a bad airbox system, it uses screws that get rusty. The more I mess with changing the filter the more damage to the airbox which is going to need some modifications soon.

My other cars just have a few clips to snap on and off and are easy. Point is some cars are much easier to do this simple job than others.

Compressed air flowing in the normal airflow direction will drive dirt into the paper, not good. Compressed air going against the normal flow (backward) will knock some of the coating on the paper off and change the filtering capacity of the paper, allowing more air and more dirt to pass by. So, compressed air is not good - period. Using a vacuum cleaner to pull off some of the surface dirt might make it look better but likely doesn’t pull out the dirt embedded in the filter paper which is what will eventually reduce airflow. Not so bad, but not worth the effort, IMO. The filter element is pleated to increase the surface area of the filter paper. As one area gets clogged there is more paper elsewhere capable of cleaning the air. Eventually all the paper is clogged with dirt and that’s replacement time.

Time rather than looks is the best way to judge replacement.