C. E. White wrote:
> "Michael Johnson" > wrote in message
> ...
>
>> A dirty filter will lower gas mileage on EFI engines too. Instead
>> of going into a long rebuttal I'll just provide a few links to some
>> credible web sites.
>>
>> http://www.fueleconomy.gov/feg/maintain.shtml
>> http://www.edmunds.com/reviews/list/...4/article.html
>> http://tinyurl.com/2a9v2
>> http://tinyurl.com/2hyeyx
>> http://www.mass.gov/Eoca/docs/doer/pub_info/dt.pdf
>
> If by credible, you mean sites that are repeating their granddaddy's
> advice, then I guess they qualify. Can you show me one that actual has
> any data to support your claim? Or at least an explanation of why a
> slightly restrictive air filter might reduce the fuel economy of a
> modern fuel injected engine? These sites are just repeating the same
> sort of maintenance information that has been out there for 70 years.
> Things have changed.

Maybe the way to do this is to find a web site that says a dirty air
filter will not decrease gas mileage on an EFI engine.

>> There are a huge number of sites stating that a dirty filter
>> decreases mileage. There is more than just a MAF reading that the
>> computer uses to determine the amount of fuel needed. Air density,
>> throttle position, air temperature etc. also come into play.
>> Excessive opening of the throttle plate on a fuel injected car also
>> tells the computer the engine is under a greater load which effects
>> how much fuel is delivered to the cylinders.
>
> Sigh, if you are talking about a completely plugged air filter, I
> agree that the fuel economy may be affected (heck it might actually
> increase since the maximum engine power will be limited). However, for
> a modern fuel injected car operated in a normal environment with a
> filter changed at reasonable intervals the difference in fuel economy
> between a "new" and "used" filter approaches zero. The same is true
> for a proper paper filter and a K&N filter. The only sensor in front
> of the throttle plate(s) is the MAF. It is called a Mass Air Flow
> sensor because it is measuring the mass of air flowing through the
> intact tract, not pressure or density, or temperature. The throttle
> position sensor is at the throttle and measures it position. All of
> the other sensors are behind the throttle plate. So think about what
> the various sensors see when the filter is slightly more restrictive.
> The MAF is measuring the mass of air flowing through the system. It
> doesn't measure pressure, so it won't be affected because the flow is
> reduced my a slightly higher restriction at the filter (we are talking
> about tenths of a psi difference or less). If the filter is slightly
> more restrictive, the throttle may need to be slightly more open to
> achieve the same power output. And I mean slightly. Throttle position
> sensors are not particularly precise. They are a gross position
> indicator. I doubt the difference in the throttle opening related to a
> slightly more restrictive filter is significant enough to affect the
> engine parameters at all. All the other engine sensors are after the
> throttle plate. They will not be affected by the restriction in the
> air filter any more than by the much larger restriction of the
> throttle plate. AND remember you have O2 sensors that feed data back
> to the PCM that is used to correct for variations in the other
> sensors. So even if the restriction of the air filter was severe
> enough to affect the other sensors, the feedback from the O2 sensors
> should allow the PCM to adjust the fuel trim to compensate.

Does an engine under a heavy load run richer than one that is under a
light load? The ECU has load tables that it uses to help calculate what
it believes the optimum air fuel ratio should be taking into account the
conditions it thinks the engine is operating under. What the computer
thinks is optimum isn't always 14.7:1. The computer expects the
operator to keep the filters fresh and a dirty air filter makes the ECU
think the engine is under a heavier load and thus changes the target A/R
to run richer than normal. This in turn reduces gas mileage. ECUs
aren't clairvoyant and can determine the degree to which an air filter
is dirty. It only takes the input from ALL the sensors and using
preprogrammed tables makes A/F adjustments (and many other) to the
conditions it perceives the engine to be operating under. Determining
engine load is a very important component is what it uses to set the
target A/F at any given moment. This is why they put throttle position
sensors on engines nowadays. The new Mustangs are somewhat different in
that there is no longer a direct wire connection to the throttle plate.
The computer senses the position of the accelerator pedal and then
sets the throttle opening accordingly.

I have a TwEECer chip in my '89 LX that lets me program almost all of
the EEC-IV operating parameters. I know load tables exist and they
affect A/F as do many other sensor readings. There is a lot more going
on in the ECU than reading air in and making a simple computation for
fuel required. Things like engine acceleration rate, load, etc. come
into play in a big way. When something like a dirty air filter causes
readings of the throttle position sensor to be out of the range it
expects for a given driving condition then gas mileage can, and will, be
affected. The computer doesn't know the air filter is dirty and tries
to run the engine in a manner that isn't optimum for gas mileage. It
thinks you are doing something like climbing a hill, or accelerating,
and delivers fuel accordingly.

The A/F doesn't remain constant across the entire load range an engine
can experience. As the load increases the target A/F decreases. This
is programmed into the load data tables of ECUs. If the A/F didn't
decrease then cylinder temperatures would get too high and start melting
things like piston tops.

To further make my point does an engine get better gas mileage going
downhill or uphill at the same speed? Is that because in one condition
the engine is under a heavier load than the other? If the rpm rate is
the same for each condition then why is the mileage different? It is
because the throttle has to be open more going uphill to get the air
necessary to make the power needed to overcome the elevation increase.
The data tables for engine load the computer uses are very specific to
the size of the throttle body put on the car. It relies on these tables
in conjunction with throttle position readings to determine what target
A/F is used from the load table. It also uses the O2 readings to fine
tune the A/F but only to meet the target A/F from the load table. A
dirty filter requires an increase in throttle opening which triggers the
computer to operate from the higher end of the load tables. Hence the
engine runs richer and gets lower mileage.

>> I'm not saying a K&N filter will give a noticeable improvement in
>> gas mileage over an OEM unit but with all things being equal the
>> engine with a more efficient filter will perform better. Do you
>> think an engine with a dirty air filter would pass an emissions
>> test? If so then why not?
>
> As long as the filter is in good shape, it will have no significant
> effect on the ability to pass an emissions test. See above for the

So if the filter is dirty enough then it will affect whether the
emissions test is passed? This means the A/F ratio isn't optimum,
doesn't it? If what you are saying is true then the computer should
compensate and make the A/F optimum thus allowing it to pass the test.

> reasons. I won't argue that a K&N might provide a slight performance
> increase at WOT. It very well might. But for anything but large
> throttle openings, the throttle plate is by far the most significant
> restriction in the intake tract. The filter is almost not there as far
> as air flow is concerned until the throttle is nearly wide open.
> Again, I am only talking about modern fuel injected engines. For older
> carbureted engines, a restrictive air filter would definitely
> significantly reduce fuel economy. And the situation is not clear to
> me if you are talking about some of the early speed density type fuel
> injection systems (systems without a MAF). For normal sorts of air
> filter restriction the PCM of these types of systems would be able to
> compensate for a restrictive air filter. However, for a very
> restrictive filter, they may not. However, as far as I know, no one
> has sold a car with a speed density only system for a decade.

You keep saying for "normal sorts of air filter restrictions" when a
restriction is a restriction. A filter just doesn't not affect mileage
one day and then suddenly becomes dirty enough to affect it the next.
It is a gradual progression that happens continuously and in most
circumstances is too slow for the driver to perceive.

Also, the basic operating parameters of an engine doesn't change because
it is fuel injected and computer controlled. The computer actually
mimics the old carburetors, governors, points etc. by using sensor
readings. The ECU controlling today's engines isn't a HAL 9000 that
thinks like a human. If it is getting garbage input from the sensors
then it spits garbage out to control the engine. The effect of a dirty
air filter on a fuel injected engine is the same as one with a
carburetor. On both engines the dirty filter puts the engine under load
and it is this that decreases gas mileage in both cases.


> By the way, the DIY Basics sight you referenced is loony
> (http://tinyurl.com/2hyeyx). You should follow your vehicle
> manufacturers replacement schedule for the air filter. I think that
> sight must be run by filter manufacturers.
>
> You should read these sites:
>
> http://www.visteon.com/utils/whitepa...05_01_1139.pdf

I searched the PDF for "gas mileage" "mileage" and "mpg" and got no
hits. It's a little too long for me to read through but it seems to
address filtering efficiency for removing particulates and not the
effect of dirty air filters on gas mileage.

> http://www.filtercouncil.org/techdata/tsbs/89-3R3.html

This also seems to be addressing filter efficiency for removing
particulates and not gas mileage efficiency.

> Air filters (paper and K&N) are less efficient at removing dirt when
> they are new. As the accumulate particles, the filtering efficiency
> improves. So changing your filter too frequently (or cleaning your K&N
> too often) can actually increase engine wear. You should also consider
> that K&N filters loads up with dirt much faster than paper filters
> (they have less dirt holding capacity). So while they may enjoy a flow
> advantage when new (or when just cleaned), the advantage decreases
> rapidly with time.

I have no doubt that a K&N filter lets more particulates by (and
therefore more air itself) than an OEM filter. Where the debate starts
is whether is has any appreciable impact on engine longevity for the
average vehicle. I believe it doesn't based on my own experience. My
'89 LX had had the same filter installed for over 130k miles and I had
the heads off at around 150k miles and saw no appreciable cylinder wall
wear. It also doesn't noticeably use any more oil than when it was new.

The air filter debate is similar to the synthetic verses conventional
oil debate. IMO, there is no appreciable difference in wear from using
any oil that is changed every 3,000-4,000 miles. The contaminants never
get a chance to build up in the oil to cause a problem whether the oil
is synthetic or conventional.