serious drop in MPG. 20-22 MPG

Well, somewhat. But what I think happens is that the prius compensates with the use of more gas. It definately made a difference when I changed the air filter. Just as it did when I cleaned out the EGR tube to the Intake manifold.

 

I've also noticed a little better mileage when I changed from 0-20 oil to 0-16.

 

Pushing in more gas doesn't produce more power when there is a shortage of air to burn it. So when running short on air, be it a clogged filter or driving up a high altitude Colorado mountain road, power will be missing.

Modern car engine controls, including in Prius, normally run a fixed air::fuel ratio for both pollution control and efficiency reasons. When the air supply is restricted, the computers control fuel to match. Maximum possible power goes down, but when operating at less power than that max -- i.e. you don't have the gas pedaled completely floored -- there is no reason for fuel efficiency to change.

In traditional Otto cycle gasoline engines (before the Atkinsonized Prius engine), the air filter restriction is in series with the throttle valve restriction. Only the total combined air flow restriction really matters. As the air filter gradually clogs up, or as the outside air thins with increasing altitude, the engine controller or driver compensates by opening the throttle valve a bit more, achieving the same power and efficiency as with a clean filter or even no filter at all. Nothing materially changes until the throttle valve reaches its open limit and can't open any more. That is the point where the driver suddenly notices a lack of needed or desired power.

The Atkinsonized Prius engine may shift things around a bit, as it is meant in part to run the throttle valve wider open and get at least some control of power by other means (e.g. variable displacement and EGR), but the overall picture shouldn't be all that much different.

I strongly suspect this to be a coincidence, lining up with something else.

 

Well, O, righty then. I concede to the idea that it could’ve been an effect of something else. Seems strange that the difference happened when the change was made. Having a less restrictive air intake increases air flow available so I would suppose a more efficient operation and responsiveness. Recently, I finally changed the egr cooler. And in doing so the responsiveness increased remarkably. The old one was 95% clogged. In giving the engine the ability to “breathe “ better, like in changing the air filter, caused my fuel consumption to decrease and responsiveness improve. So, when you change the air filter, even irregularly, doesn’t hurt and actually helps with the overall efficiency including the cats and what the O2 sensors send to the ecu. Saving fuel and increasing mileage. I still think that there is some sort of systemic change that isn’t readily apparent. Things like the 12volt system charging rate changing with ambient outside temperature for example. So, maybe it’s just haunted.

 

Note that I was challenging only the efficiency (MPG) side of the claim, not the responsiveness (power) side.

If you had a diesel engine, which is meant to operate with no intake restriction, you'd have a stronger point with respect to efficiency.

But gasoline engines are basically built around the idea of restricting air intake as their primary means of power control. A completely unrestricted intake gives a screaming runaway engine. You use the accelerator pedal to control the amount of throttle restriction. A small or even medium amount of air filter restriction in series with that throttle is inconsequential, up until the point you really want a screaming full-power output. Then that clogged air filter just means you lose the race to the other guy who can get more power because he has a less restricted filter. But the majority of us are not actually racing out on the streets.

You don't want to run with a 95% clogged air filter either, I doubt that it will even idle well, let alone get up to highway speed. But lesser restrictions that still give you all the power you want when accelerating to highway speed, should have almost zero impact on MPG.

 

I’ve read that if the engine air filter is clogging the car’s computer(s) will reduce gas being injected, with aim of keeping proper fuel/air ratio.