2002 a4 air filter change -- problems

i put in a "supposedly" OE air filter made by mahle a couple of months ago. i noticed at the time that the filter was slightly larger than
the one i was removing (which was original stock equipment). i put it in anyway, since the difference in size was less than a centimeter all around.
first of all, do air filters shrink a bit as the age? if so, that might explain why the difference exists.
secondly, and this is more worrisome -- since i changed the air filter (i also put in four bosch iridium spark plugs), my gas mileage has dipped by about 5 mpg on the highway.
could that "too big" air filter be causing air flow problems?
also, my slightly choppy engine idle, which was present before the changes, is still there, but a little less noticiable -- you might call it "intermitent light chop" in airline terms. minor, and not always present.
so the engine is running a bit more smoothly, but is not performing as well in terms of fuel efficiency.
any ideas or suggestions?
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of snugness in the fit so that the endseals can operate in their correct manner to ensure that all intake air is drawn through (and not around) the filter. Was the dimensional difference between the old and replacement filter large enough to prevent proper sealing to take place. If you remove the new filter again, if it has been in the housing for a few days you should be able to see "witness" marks on it to show if it is seated properly in the housing. If you are operating the engine in significantly dusty conditions an improperly fitted filter will reduce the engine service life as a consequence.
I assume that your engine is fuel injected and this will afford some measure of self-regulation in the fuel air ratio, assuming the inlet manifold pressure is higher due to the ill-fitting air filter. This tendency of self-regulation of the fuel/ air ration will be less marked in the case of a carburettor metered mix.
An over lean mixture can also result in reduced mileage, with less power resulting in increased throttle openings to maintain anticipated speed.
The colour of your spark plug insulators may give you a coarse yardstick as to the mixture correctness.
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You changed two variables at one time and, unfortunately, now have a problem that might have been caused by either. So my suggestion is to change one of the variables back and see what happens.
More than likely, it's not the air filter. Also, more than likely, your old plugs are in a landfill someplace.
If it's any comfort: it's never happened to anyone else :-)
Ken
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