Modern Engines Work Differently

Dear Car Talk:

Since I first poked my head under a hood nearly 50 years ago, the cleanliness of the air filter was always one of the factors that affects fuel mileage. I still hear it today, even though modern engines are controlled by computers.

Since the computer and its associated sensors constantly maintain the optimum fuel/air mixture, it seems that a dirty air filter (within reason) would have little to no impact on fuel economy. You may need to use a little more throttle to maintain a given speed, but wouldn’t the fuel economy be essentially the same? — Scott

Yes, it would.

Obviously, if someone dumps a pot of five-alarm chili in your air filter, all bets are off. But up to that point, it hardly matters.

Here’s how modern engines work: Air coming into the engine passes through a filter. That weeds out pigeons, leaves and other dirt and debris. The air that gets through the filter then goes past a sensor called a mass airflow sensor. That sensor measures the amount and temperature and, as such, the density of the air that’s coming in.

Then, based on that information, the computer decides, many times a second, how much fuel to inject to make the ideal fuel/air mixture in the cylinders. If the amount of air gets reduced over time due to a dirty air filter, the computer will simply adjust and send in less fuel to match it. So your mileage and, just as important, your emissions, will stay the same.

If it gets really plugged up, it could affect the amount of power you get. But it won’t affect anything else. And honestly, we just don’t see dirty air filters like we used to. Twenty-five years ago, an older car might come into the shop, and we’d pull out the air filter, and it’d be disgusting. It’d be greasy and oily and almost black. Now when we see a really dirty air filter, it’s usually because some rodent has been using it as a place to warm its nuts.

The reason air filters stay cleaner is largely because crankcase ventilation systems work much better than they used to. In the old days, they’d send oily fumes through the filter and make it filthy in no time. Plus, outside air, in general, is cleaner. That’s a great accomplishment, and not just for cars. So if you check your nose hairs, Scott, they probably don’t need changing as often either.

Todays Car-o-Scope

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