Engine Air Filter, why change


#1

Every time I go for an oil change I am told I need to a new air filter. I always say no, because unless you are using full throttle it does not matter. In the old days of carburetors the fuel coming into the engine was determined by throttle opening and a partially blocked air filter would cause a rich mixture. However, on fuel-injected engines, the fuel quantity is determined by the airflow downstream of the air filter, and is always correct. In order to get a certain performance it just needs a little more throttle opening. Do I have wrong thinking on this?


#2

In order to get a certain performance it just needs a little more throttle opening

Well, it will eventually get to a point where it’s not taking in enough air to get you the power you’re requesting by giving it more throttle. The computer will see less air coming in, so it’ll inject less fuel, giving you less power. Generally speaking unless you live in dusty environment, you can go 20k miles on an air filter before it gets dirty enough to impact performance, sometimes more. But if you live an area that gets a lot of dust/dirt in the air, then you’ll be replacing them more frequently.


#3

Your thinking is correct. At least I think it is. A dirty air filter on newer cars would be compensated for by the computer, which would just inject less fuel for a given throttle position. But there are other parameters involved in the computer’s fuel/air calculation too. So one of them might get affected and cause some problem even at lower than max throttle. Like the MAF – which measures the air flow at a single spot – assumes a certain air flow profile through the air intake chambers to calculate the actual total air flow, and a dirty air filter could perhaps change that. If you look at the air intakes of modern cars, you’ll often see weird looking appendages attached, some look like bananas, etc. Those are to insure the air flow profile matches what the MAF assumes.

I change the air filter in my Corolla every two years, and that seems to work for me. If you drive on dirt roads where a lot of dust gets kicked up, common sense would say it is best t o change it more frequently. But probably not necessary at every oil change. That seems excessive.


#4

Fuel mileage will not be affected by a dirty air filter on a modern car. That is correct.

When is the last time it was changed? Chances are that the oil changers are pushing something that doesn’t have to be changed as often as they like. However, it does need to be changed periodically.

I mean I don’t have to put on clean underwear every day, but I always do. I could stretch it, but just because you can do something doesn’t mean you should.


#5

It’s too bad you can’t change an oil filter like you change your underwear. Turn underwear inside out and use them a second time - turn an oil filter inside out and use it a second time.


#6

@Derek1935,

This government study agrees with you:

Even most of the old carburetors didn’t change MPG much with a clogged filter, because they use the air pressure after the filter as a reference.
Air filters (and most other filters) get more efficient as they load up.


#7

I must add ford must agree with the OP. They have a “lifetime” air filter on 05-07 ford focuses.


#8

Every time I go for an oil change I am told I need to a new air filter.

Sounds like you frequent the quick-lube type places. Their business model is to make money off of selling you extras, often using inexperienced personnel.

Unless you normally drive in dusty areas, you should be able to go extend your changes to the mileage listed in your manufacturer maintenance schedule. It’s likely 30K or more. If you want to go further, hold it up to the sun and if you can see daylight through it, put it back in for another 5-10K miles.


#9

Air filters eventually deteriorate and crumble, allowing dirt to enter the combustion chamber. Changing them every 2 years is likely adequate for highway driving unless the humidity remains very high. And I would discourage using air pressure to blow back through an air filter to clean the paper.


#10

“And I would Discourge using air pressure to blow back through an air filter to clean the paper.”

You do know that they make filtration systems that do just that?

http://www.fredfiltration.com/technology.asp

Tester


#11

When the nozzle is 15 inches from the filter media as mentioned in the link, damage might be avoided. And the link seems to be marketing a device for cleaning the large, expensive filters used on diesel trucks which cost hundreds. A fleet might consider a special machine to clean such filters worthwhile. Of course those filters are considerably more heavy duty than passenger car air filters.

I took a second look at the link and the filters referenced were industrial filters. I might guess that they are considerably heavier than automotive filters costing less than $5.00 each.


#12

I’m saying you can use compressed air to clean an air filter. I do it all the time.

As mentioned, air filters become more efficient the dirtier they get. So if the fine particulate is impinged into the filter media, and you use compressed air to blow off the larger particulate, the filter flows better but also filters better.

It’s when you blow the filter off and you can’t see the media it’s time to replace the filter.

Tester


#13

A cheap, damaged, or overly dirty air filter can allow fine dirt to be sucked into your engine, either through the filter, or by air being sucked around it. This can cause initial damage, but also contaminate your oil, causing higher than normal wear. Yes, the oil filter will get some, but some will continue to circulate. I have had oil analyses done on my cars and read reports from others—these contaminants are detectable, and show up on the reports. I have personally had one that was higher than normal in silicates (sand), where a dirty air filter was suspected. Sure enough, my old air filter was a lot dirtier than I’d thought. So maybe something to consider as well.

If a place is trying to get you to buy a filter at every trip, either ignore them, or preferably, start going to places that aren’t “Jiffy Lube” type places. I used to go to those places and have had them try to upsell me on an air filter that I’d just changed less than a month prior–sounds like your story.


#14

If you never check the condition of your air filter, you don’t know what condition it’s in…A backfire may have ripped it apart or a rodent may have chewed it apart or the grease monkey that changed it last may have not installed it incorrectly and it’s bypassing…The time may come when you NEED every bit of power your engine can produce and a filthy air-filter will greatly diminish that power. I change mine every third or forth oil change…


#15

The lifetime filter is lifetime only because it is sealed inside the housing.When it becomes clogged you have to spend hundreds to get a new housing and filter.
Sort of like the lifetime lubricated ball joints and tie rod ends used today. Their lifetime is only the lifetime of the grease in them, when it is gone they are gone. I keep my cars a long time and I never had to replace a greaseble front end component.


#16

There are many reasons to change an air filter, as posted above. It also may have gotten wet, the outer seal may be damaged, it may be oil contaminated from a poorly operating breather system. Replace it when it looks dirty. What’s it going to cost, maybe $15?


#17

Most quick oil change places try to sell a new air filter to just about every customer, and this is simply an upsell for additional revenue and profit. Most current cars recommend replacing the air filter every 30K miles which for the average driver is 2 to 3 years. Just say no to the guy showing you your “dirty” filter and drive on.

If you live on dirt roads you might need to replace a filter sooner than 30K, but most of us can stretch the interval beyond 30K without an issue. I simply buy and replace my air filters every couple of years myself and say “no” to service writers who want to sell me one.

As a filter ages it will become a better “filter” but it will also start to be more restrictive of airflow. An analogy is the filters on a home vacuum. As the filter gets more restrictive less air moves and suction is reduced. Eventually the forces can rupture the bag and you have a big mess. A filter in a car that gets too restrictive will be out of range with the specs for the car and can affect performance in some way.

There really is no good “home” method for testing air flow, so replacing periodically is the best option. You really shouldn’t have any problems until you go past 30K miles however. So, too frequent replacement is simply a waste of money with no benefit.


#18

The recommended change interval is listed in the owners manual and that is what I go by.


#19

You are absolutely right, howerver if you want to retain emergency power, your owners manual is a good guideline.


#20

The computer makes up for decreased air flow, that is true. A car is like a person, it need to inhale and exhale. If either is restricted performance is decreased. Put a wet rag over your mouth, then try to run. Makes it a bit harder. Your car is the same. I change mine every 10k miles and always after Ga pollen season.