Modern engines work differently

Ray Magliozzi

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

RAY: 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.

Reliability isn’t everything

Dear Car Talk:

I just traded in my 2004 Jeep Liberty for a “used” 2020 Jeep Renegade Latitude with 4,000 miles. It was a demo. Sticker price was $29,000. I paid $21,000. It has all the bells and whistles; cold weather package, safety package, etc. I even got the color I wanted: Metallic Bikini.

My son is a mechanic. He was, well, let’s say, unhappy with my choice. He said, “Mom, it’s a Dodge Neon engine in a Jeep body.” I love my Renegade. I joined the JROG (Jeep Renegade Owner’s Group), and it seems most people love their Renegades.

Am I an idiot? Is my son right? Did I make a bad choice? I did get the extended bumper to bumper warranty because of the amount of electronics. Please give me some ammunition to use against my son. Thank you. – Gretchen

RAY: Your best ammunition is “your mom is happy, son.” I mean, he’s not wrong. The Renegade would not be my first choice, in terms of reliability or advanced engineering. On the other hand, my mom never wanted a car in Metallic Bikini.

He’s probably worked on cars in a shop and seen more than his share of Jeeps come in, and he’s just looking out for you.

I think your other strong argument is that with all the money you saved, you wisely bought the extended warranty. So if something goes wrong with the Jeep, tell your son that he won’t have to scrape his knuckles to figure it out and fix it. You’ll be able to go back to the dealer and get it fixed for free, while fending off the salesman trying to sell you another Jeep.

You also might tell your son this: Reliability isn’t everything. There’s also fun. And cute. And the image of adventure. And being in love with your car. And some folks are perfectly willing to trade off some reliability and lots of other things for those characteristics. Hey, it’s a free country.

Plus, if you’re stepping up from a 2004 Jeep Liberty, this Renegade has got to feel like a Mercedes S-Class to you, so tell him to let you enjoy it. And don’t tell him when it breaks; just quietly get it fixed.

Got a question about cars? Write to Car Talk write to Ray in care of King Features, 628 Virginia Drive, Orlando, FL 32803, or email by visiting the Car Talk website at

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