Air filter and fuel economy

I frequently see dirty air filter listed as a major factor (since it
is listed in the first few items) in degrading fuel economy. Doesn't
this just simply restricted the rate of air input and the ECU would
then adjust the fuel flow to suit?

I'm currently looking for the factors that has caused a sudden drop of
~25% (from 6.5km/l to 5km/l) in fuel economy in a 760 V6 and the above
assertion puzzles me. In my case, as the oxygen sensor is making
enough transitions between high and low value, the fuel mixture is
probably ok. In my way of thinking this means the air filter and the
air way is no longer a major factor. The poor fuel economy is caused
by un-burnable fuel going through the system due to: a) Weak spark;
and b) Fuel spray pattern or fuel droplets size. Any idea how to test
for these problems?

The other factors I can think of is drag due to things like brakes and
fuel leaks, which I have found and corrected. Can anybody add
something else?

 

True.

It does several things. One is to reduce the efficiency of exhaust
scavenging (and thus economy).

Although European "performance car" nuts in past decades would tweak
the air filter as one of the first upgrades, they were often running
the sort of long-overlap camshafts that were least sensitive to
restricted airflow. Unless they ran their engine completely flat out
(which admittedly you had to back in the days of anything over 100HP
being a big engine) it didn't really make any difference.

These days, with effective engine management systems, a dirty air
filter often shows up first as a dirty exhaust, rather than a loss of
economy.

 

More possibilities:

Colder weather, shorter trips, oil change to a more viscous weight of oil,
more headlight and other high electrical draw use (heated seats, electric
defrosters, higher blower speeds, etc.).

 

A frequently overlooked reason is the engine running colder (not at
operating temperature) caused by a faulty engine thermostat.
Joe

 

I would suspect the O2 sensor before anything else. If it is failing (or
has failed), the ECU drops into a default over-rich condition which will
adversely effect your mileage. If you know that the sensor is good, then
grab a can of electrical contact cleaner (Deoxit is an excellent cleaner)
and start on the sensor connections under the hood. A faulty coolant
temperature sender will keep the ECU from going into closed loop and reading
the O2 sensor as well.

Good luck,
John

 

Since the V6 is normally aspirated, there's no turbo to help in pulling
air through the filter system. I wonder what loss could be laid to what
is generally called "pumping loss" of power. I suppose that a bad
filter would degrade mileage in a turbo, too, in that it would have to
work more of the time.

Just a thought, but with modern engines automatically compensating for
just about any/everything, the system response would be to provide added
fuel for the added work that air problems require.

bob noble
Reno, NV, USA