Clean vs. Dirty Air Filter MPG.

Discussion in 'Fuel Economy' started by Pirate, Jun 25, 2013.

  1. Pirate

    Pirate New Member

    Always wondered about this. If the computer in a car adjusts fuel ratio, when the air filter is dirty, and passing less air, and the computer is allowing less fuel, how does this effect mpg?
    I would think it would be like having less hp, and having to press harder on the pedal.

    On my 2002 Taxoma V-6, with 140,000 miles, and having only changed the air filter 3 times, the mpg. hasn't changed. (Scan Gauge) This is with the orig spark plugs also.
  2. ALS

    ALS Super Moderator Staff Member

    In today's computer control vehicles they adjust to the lower air flow associated with a dirty filter. A clean air filter every 30K just makes the engine work a little less. With an old engine with carburetor a dirty air filter made a pretty big difference in fuel economy.
  3. PaleMelanesian

    PaleMelanesian Beat the System Staff Member

    Consider what part of the engine's operating range you're using. It's built to go Wide Open Throttle at max rpm (7000?). If you're at part throttle and 2000 rpm you're drawing in maybe 15% of the max airflow. A filter might be a slight restriction at max, but in normal operating mode it's insignificant. The difference between a clean and dirty filter is even smaller than that.
  4. Mendel Leisk

    Mendel Leisk 2010 Prius (CAN Touring) Staff Member

    ^ What they said.
  5. phoebeisis

    phoebeisis Well-Known Member

    Like they said-no difference-since total intake restriction at part throttle is adj by ECM- or your foot
    More before the throttle plate restriction-means throttle plate will be opened more-by you or by fly by wire ECM

    Same reason CAIs won't improve part throttle FE
    Now 140,000 on plugs-probably worth changing-but you won't be able to detect better FE
  6. worthywads

    worthywads Don't Feel Like Satan, I am to AAA

  7. RedylC94

    RedylC94 Well-Known Member

    Yes, a partially clogged air filter will definitely reduce power, but should have little to no effect on fuel consumption of modern vehicles.
    True, if it's badly clogged, although I've never been able to detect any difference in fuel consumption or power after replacing filters (or plugs) with well over 100,000 miles in my carbureted car. That obviously would not be true if I drove as much in dusty conditions as some folks must. Since dust and particulate levels vary widely, the most relevant factor in determining when to replace a filter is pressure drop across the filter, not mileage.

    Changing air filters excessively frequently will slightly increase the amount of dirt that gets through, since a little dirt helps block subsequent small particles that might find their way through a new filter.
    Last edited: Jun 26, 2013
  8. The Doctor

    The Doctor Old Dude, New Car

    Many drivers today, have no history with automobiles. They've never known anything but computer controlled engines, with fuel injection, DFI, VVT and all that stuff.

    They've never even seen an 'Oil Washed Air Cleaner'. They worked very well, but were a real Bi#^$ to clean.
    Service Station mechanics really hated those things.

    The paper 'drop in' filter was not an improvement, it just made changing filters a lot easier. So many drivers today think that a paper filter is the ultimate filter, because that's what comes in all new cars today.

    But to pass enough air to keep the engine running, the filter will also pass a certain amount of very fine dirt. It's called a "Trade Off". Cheaper filter=more dirt in the engine.

    It takes more power from the engine to suck air through a dirty filter than a clean filter.
    So for both my money, and the health of my engine, I use the K&N 'oiled fabric' air cleaner and I wash it and re-oil it every 10k miles. But, every 3k I take it out, knock out the bugs, leaves, other debris and check the color. If it's pink, I leave it, but if it looks like it's brown, like it's been dipped in mud, it gets a wash and re-oil.

    It's not rocket science, in fact it's very Old School, that an oily surface will trap more fine dust particles than a dry paper surface. :)

    Cheers Mates!
  9. ALS

    ALS Super Moderator Staff Member

    The Doctor, I had a K&N in my old Volvo wagon and learned the hard way about those wiz bang filters. At 247K miles my turbo suddenly seized. When I pulled the inlet hose off the Turbo the impeller was covered in oil and dirt. I had been using the K&N for a few years and thought it was also better than paper. At that point I switched back to paper and tossed the K&N in the garbage. When I traded in my Volvo at 370K miles I pulled the inlet hose again just for curiosity. The inlet impeller looked brand spanking new even with 123K miles on the turbo. There was $800 I probably wouldn't have had to spend on the car had I stayed with a paper filter.

    There are people who swear by them, I'm not one of those people.

    If you are replacing your car or truck every five or so years it probably would be a problem, but I keep cars for over 200K miles and I do see a problem with an oiled filter over the long haul.
  10. The Doctor

    The Doctor Old Dude, New Car

    It's a well known and even documented fact, that some folks just don't know when enough is enough.

    There is a proper way and an Improper way to re-oil any brand of oiled element air filter.
    K&N is not the only company that makes them.

    It's kind of like Brylcream.... a little dab'll do ya!

  11. worthywads

    worthywads Don't Feel Like Satan, I am to AAA

  12. ALS

    ALS Super Moderator Staff Member

    Worthywads, there is also a test over on Bobistheoilguy and the K&N was at the bottom when it came to the dirt test. It did flow the best but you have to give up filtering for flow.
    In our use where we are striving for better fuel economy I'll take better filtering over a miniscule increase in air flow.
  13. worthywads

    worthywads Don't Feel Like Satan, I am to AAA

    2000 rpm max 98% of the time here. I decided to go 60K intervals on my OEM Toyota filters.
  14. oilburner

    oilburner Active Member

    I buy factory honda filters for this reason. They look like paper type that filter really well with a layer of oil on top. They are definitely not dry.
  15. briando

    briando Member

    how much does a dirty air filter really affect gas mileage. i'm trying to keep my prius as low maintenance as possible. help me out
    Last edited by a moderator: Aug 20, 2013
  16. worthywads

    worthywads Don't Feel Like Satan, I am to AAA

    Last edited by a moderator: Aug 20, 2013
  17. xcel

    xcel PZEV, there's nothing like it :) Staff Member

    Hi All:

    A dirty air filter will not affect mpg one iota up to the point of air starvation. The throttle plate opens further to produce the same pressure drop from outside to the intake. Once it is wide open however, that is where you will lose a lot of power and the pumping loss cannot be made up with a wide open throttle plate.

    Follow manufacturer rec's and if you live in a Phoenix dust storm every day, change it in half the time or less!

  18. phoebeisis

    phoebeisis Well-Known Member

    The K&Ns killed many mass air sensors
    and it really isn't clear just how much oil is enough
    and how much is too little??

    Do you actually weight your filter before and after oiling??
    Or measure out a specific quantity of oil?
    Or do you just eyeball it like everyone else-and hope for the best-
    "looks about right" is fine with hair-
    maybe not with a $25,000 car.

    Modern car engines-almost NEVER fail before the rest of the car turns to scrap- so cheap paper filters are obviously adequate.
    There just isn't much upside-but plenty of downside.

    I wonder what race cars-maybe rally cars-use?? Granted they don't have to last 200,000 miles
    What sort of filter does GM require on the high end Corvette- - or Porche or Ferrari ? Performance is everything on those cars, and they are generally maintained by dealer mechanics so if K&N was better......?
    Last edited: Aug 28, 2013
  19. The Doctor

    The Doctor Old Dude, New Car

    I've heard, or read, every sick argument against the K&N Air cleaner, yet, I've used them since the 70's and have never had one moments trouble with them.
    I've used them on motorcycles, family cars, 4x4's and SUV's. I'm currently running one on my 2013 Kia Soul. I gained 3mpg when I put it ON.

    Granted, the one servicing the filter does need to be able to read plain English and follow simple instructions. It's not rocket Science!

    Most of us who successfully use the K&N air filter, can read, and can follow instructions and we report a ~3 mpg improvement in performance.

    Most of those CAI's the guys rave about have a pink filter on the end of the CAI. That' almost always a K&N Air Cleaner.

    I'll keep mine, thank you!

    The Doctor :cool:
  20. EdwinTheMagnificent

    EdwinTheMagnificent Legend In His Mind

    I've used paper OEM filters on every car , and never had one problem with them.

    And if the CAI people want to use the K&N , that's a good reason for me to stay away .

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