On Thu, 19 Jun 2008 14:35:02 -0500, Steve > wrote:

wrote:
>
>>
>> A DIRTY AIR FILTER WILL NOT AFFECT NORMAL DRIVING FUEL ECONOMY ON
>> TODAY'S MAF (Mass Air Flow) sensored vehicles.
>
>Yes and no. If the MAF were the ONLY sensor that affected mixture, then
>the above would be true. But the actual fact is that mixture is set by a
>combination of throttle position, MAF, O2 sensor, and other parameters.
>When the air filter gets extremely dirty, the throttle will have to be
>cracked open more than "normal" to get a particular power from the
>engine, so the MAF and throttle position sensors will "disagree"
>slightly, meaning that the calculations the computer does to determine
>the fuel injector duty cycle may be slightly skewed.
>
>So there IS an effect, but its probably too small to measure in
>real-world conditions where changing temperature and humidity (among
>other things) would easily mask the fuel penalty.
>
>Of course the dirty air filter will reduce peak power available, and it
>will still cause the mixture to be significantly rich during open-loop
>operation where the MAF sensor is ignored.


Open loop is usually VERY short. On my car it's about 20 seconds.
(80F)

>
>And speed-density systems (MAP sensor instead of MAF) will be affected
>differently than MAF systems.
>