If you’ve ever thrown a couple of rev’s at your engine with the hood open, you’d see how much it rocks back and forth. Multiply that by 20+ years of driving combined with under-hood heat and you’ll understand how these develop cracks in them. When cracked they let in un-metered air causing rough run due to running lean. Check for the cracks in the “low spots” of the accordion (especially on the underside). NOT an aftermarket counterfeit, OE part that is built to last.