Piston Slap: NVH Nightmares No More?
October 18th, 2019 3:17 PM Share
Doug writes:I have a 2007 Mercedes-Benz S550. I accidentally bought it in 2014 off eBay (long story) for about trade-in ($13,000), with 150,000 miles on the odomater. In a twist of good luck, it turned out to be a one owner car and using CarFax I was able to see and verify that it had been maintained by the selling dealer right up to a few weeks before its eBay appearance. A call to the dealer confirmed the complete service history. Even the brake pads and rotors were new, and it had a newish and very expensive set of Michelins. Almost three years later, it has been very reliable for my teen daughter and has 180,000 miles. It had a few quirks I have fixed myself (clogged charcoal canister, minor trunk leak caused by a missing rubber plug, sticking power driver seat) and only one real repair (dead stereo amp rebuilt by Becker).Now that my daughter is off at college and content with a ZipCar, I am driving it and intend to keep doing so. I have noticed a vibration in the center console area of all places, while driving around town. You feel it through your arm resting on the console lid. It’s a deep vibration, if that makes sense — coming from under the car.You don’t feel it in the seat, nor do you feel it in the steering wheel. You do not feel it at a standstill, just at 20-30 mph. It goes away at higher speeds, and the car is rock solid and smooth as glass at 75 mph. I am stumped as to the cause, and with the age of the car, I don’t want to set a dealership or even an indy shop loose on it without more of an idea of the cause. I was thinking maybe a motor mount, but it seems like I would feel that all the time, and especially at idle, which I don’t. I was wondering about maybe some kind of driveshaft or transmission mount or connection point, but it seems like a fault that would get worse with increasing speed instead of going away. Do you have any ideas?
Sajeev answers:Dumb luck or no, good on you for finding a cherry W221 S-class. These seem significantly more cost effective than the W220 ( don’t take my word for it) so, faint praise aside, fixing this NVH issue isn’t a waste of money.This problem is likely a driveline issue, either the driveshaft, the transmission and/or differential mounts. With your email’s signature file in mind, your career likely gives you neither the time nor the knowledge to make this a DIY repair. Either you pay someone to throw parts at it (good luck) or you find a shop investing the coolest NVH monitoring tools. Time to find such a shop, or even break tradition and take an out-of-warranty S-class back to the dealership.Your symptoms remind me of when I had (slightly) deformed differential mounts on my 1995 Lincoln Mark VIII, but no way in hell am I making that recommendation. NVH troubleshooting is not for the faint of heart.I am confident you’ll pay more for a proper diagnosis to pay less overall.[Image: Shutterstock user Joyseulay]Send your queries to firstname.lastname@example.org. Spare no details and ask for a speedy resolution if you’re in a hurry…but be realistic, and use your make/model specific forums instead of TTAC for more timely advice.
#Differential #Driveline #Driveshaft #Engine #Mount #NoiseVibrationHarshness #NVH #Picoscope #Transmission #W221
Published March 21st, 2017 8:00 AM
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- Ed That has to be a joke.
Yep, driveshaft rubber flex damper(s) are the likely culprit. There is usually one right under the driver's right elbow if it's resting on the center console. Most Mercedes and BMW RWD/AWD have a multi piece drive shaft. There are usually needle bearings at the shaft joint(s). The car in question is at about the miles where those bearings wear out causing vibrations. Find a driveshaft shop that is experienced in Mercedes. A-number-1 No-No is to disassemble the driveshaft without marking the relationship between the parts. It's very difficult and time consuming to get them back together correctly. Most experienced techs will tell you to scrap the drive shaft and buy a new one. BIG $$$.
Sometimes you just have to put it up on the rack and give it hell. A bent wheel or flat spot on a tire wont be felt at low speeds, and at high speeds, the wheel velocity hides the defect. Ever notice on washboard roads, when going over 50 mph everything smooths out?