Michael: I’d never heard of air extractors behind the rear bumper before, Michael. So after I read your letter, I moseyed next door to my friend Leo DeLeo’s autobody shop and asked him.
“Oh, sure,” he said. “I’ll show you.” He takes me over to an older car in his shop with its bumper removed and points to the back corners under where the bumper had been. And lo behold, there are two small sets of flaps!
They have several purposes. Primarily, they release air from the car when you close the doors, so you don’t build up too much pressure. Modern cars are well-sealed — for noise reduction, comfort and fuel economy — and that means there’s no easy way for air inside the car to escape.
So, if you get in the car and pull the door closed and the air pressure goes up, that can be uncomfortable for, say, your ears, like when you’re in an airplane. It can also make it hard to close the door or make it hard to close the door easily. In fact, some years ago, BMW had a system that automatically lowered the window a skosh as you closed the door, and then immediately closed the window again once the door was shut. I assume that was another way to address the air pressure.
The other purpose of these flaps is to increase the flow of fresh air through the vehicle. The fresh air comes in the front, and as air escapes out back through these flaps, fresh air moves more easily throughout the car.
When I went back to my shop to share this epiphany with the other knuckleheads I work with, one of them said, “Oh yeah. Actually, I remember once I rested a huge iced coffee on top of the sunroof as I was getting out of a car, and when I shut the door, there was so much pressure in the car, the sunroof actually lifted up a little bit. It sent iced coffee everywhere.”
Almost all cars now have these air escape flaps hidden behind the rear bumper. I don’t know if that’s how the mice got into letter-writer Doug’s car and devoured his peanut M&Ms. I still think it’s more likely that they sought out the leftover heat from the engine and found their way in from the cowl. But it’s certainly possible they used the back door. If we find a Chilton’s Repair Manual in their nest, open to the bumper removal page, then I’ll concede the point, Michael. In the meantime, thanks for the education.
Got a question about cars? Write to Ray in care of King Features, 628 Virginia Drive, Orlando, FL 32803, or email by visiting the Car Talk website at www.cartalk.com.
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