Why is my truck fluttering and bucking?

John Paul, AAA Northeast's Car Doctor, answers a question from a reader whose truck is behaving oddly when he accelerates.

The 2018 Chevrolet Silverado 1500.
The 2018 Chevrolet Silverado 1500. –Chevrolet

Q. I have a 2018 Silverado 1500 with 27,000 miles. Recently the truck has started to flutter and buck slightly when accelerating. I really notice it more on the highway at 50-70 miles per hour, probably because the roads are smoother. If I lift off the accelerator then press it normally, it flutters for a bit then stops when cruising. I brought it to the dealership, and they said it was bad transmission fluid. They performed a “triple flush,” but driving home from the dealership it still happened as before. I should add that the only thing different is that a month before the issue I changed the wheels and tires out from the stock 18-inch wheels to 20-inch Silverado wheels. I did this for the better look, but felt the overall wheel/tire diameter would be the same. The old wheels/tires were 265/65/18 and the new wheels/tires are 275/55/20. Could this be the problem?  

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A. The wheel/tire switch shouldn’t make a difference. The tire sizes are really close. I don’t see that as a problem. There is less than a quarter of an inch difference in overall height and revolutions per mile is within 10. At this point, I’m wondering if you are feeling the cylinder shutoff system coming on and off. I recently drove a 2020 GM vehicle that would switch between four-cylinder and six-cylinder operation and noticed a slight vibration at similar speeds as your truck, when dropping to four-cylinder mode. In the vehicle that I drove, I would consider it a characteristic rather than a problem. At this point, I would go back to the GM dealer and see if there is a software update to try to eliminate the condition.

John Paul is AAA Northeast’s Car Doctor. He has over 40 years of experience in the automotive business and is an ASE certified master technician. E-mail your car question to jpaul@aaanortheast.com.

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