Any theories why a slightly clogged engine air filter would yield lower HC emissions compared to a new air filter? In going over my past Corolla emissions reports, the one parameter that seems to have the most effect is the age of the engine air filter. Retaining the filter that has already been used for two years used produces lower HC emissions than installing a new one. But why?
It should work the other way around… restricting airflow should increase HC.
But then I deal with cars built this century! 
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lol … maybe you are on to something tho. The Corolla doesn’t use an MAF to measure intake airflow. Instead it uses an MAP & throttle position sensor. And it only uses one O2 sensor, pre-cat. The amount of gasoline injected is proportional to intake airflow and the O2 sensor reading. Maybe a slightly clogged air filter somehow makes the computer think there is less air coming in, so it injects less fuel?
I wonder if the O2 sensor would have something to do with it? 
Tester
That is one of the parameter’s I’ve changed over the years; a replacement O2 sensor had no effect on HC.
Tester
I have no idea and never had a test done. My first thought was if you restrict air through the filter, it may try to pull air in from some other source.
That’s a good idea, definitely worth considering. The reason it might do that is b/c the intake manifold pressure would be lower (used vs new filter), and that is where the MAP is located. So the MAP reading would be different for the same airflow, but with a used filter.
How clogged is a “slightly clogged” air filter? If it’s only slightly clogged it should have zero effect on air flow except at full throttle / very high engine speed. Is that how you’re measuring your HC emissions? At full throttle / high engine speed?
Even most carburetors compensate for intake restriction.
It’s the state’s test method, not mine. They drive the vehicle on a car-treadmill at around 25-35 mph mph I think. So there would be significant air flow, but far from maximum.
Thanks for posting the interesting article. Unfortunately they didn’t test for HC emissions. But they claim the air/fuel mixture is not affected by a clogged air filter for modern fuel injected designs, so it seems reasonable there may be no effect on HC emissions. At this point I guess we don’t know. Not sure if any of the vehicles tested matches my Corolla’s MAP-only configuration. That could make a difference as well. Seems like it would be easier to compensate with a MAF.