I recently purchased a 2001 S500 with 79k miles. When it starts, it runs 100% flawless and quiet, except for a small vibration like a hesitation, kind of like a pulse through the vehicle at idle. If you are sitting still, you feel it through you ever few seconds and it only lasts a second or so.

My first thought was motor mounts, as I know many vibrations are that. That is also the best way I can describe the vibration. It is VERY SLIGHT, but you notice it's there, kind of like a bad or rough idle. It's quiet, so you cant pinpoint under the hood either, but if you hold your hand on the engine, you can feel it, again, very slightly.

I opened the hood and revved the engine while in neutral and park and the engine doesnt move! So now I am convinced it is not motor mounts.

Any other ideas? It is not there or unnoticeable driving, it's only at idle.

To check the motor mounts, you power brake the car in reverse and drive. Not park and neutral...that will put no load on the mounts.

My 04 S500 does this as well, would love to find a solution. I'm doing plugs next week.

I am thinking plugs too... I will check the reverse to drive and see if the engine movies. It only has 79k on it, but its a 2001 and I have no idea if plugs are original or not. Any recommendations on plugs, and also plugs+wires or just plug changes?

Make sure you buy 16 of them

yep - Thats why I am trying to narrow down issues. I dont want to buy junk, and I am NO mechanic. So I will be paying someone for the install, but Im sure I can find the plugs on eBay. Just not sure if I need the plugs and wires...

if you are paying someone to install , are you using a shop or backyard mechanic. Use a real shop and let them diagnose and buy the parts. Backyard repairs by someone without MB experience will result in a car that runs worse than now.

Besides the engine mounts, transmission mount is a very common failure item that results in vibration

It's the plugs/wires.

Also don't buy anything on ebay.

"Das Geld 2 - When you say dont buy on eBay, isn't it ok to buy a new set of NGK plugs in the box or no? From what I have seen, it would roughly cost me $100 to buy 16 plugs on eBay, depending on the brand and type. I am not doing performance upgrades to this car, so I doubt I need anything crazy expensive, but I want the car to run and last as long as possible. Recommendations?

Everything on Ebay is fake.

Try AZautohaus
Partsgeek
RMeuropean
RockAuto

Agreed. Brand name parts on eBay are often fake

i also have vibration but it only seems to affect the back which makes the back shake including the shade could this be trans mounts or could the engine mounts do that to?

Probably trans mount

To check the motor mounts, you power brake the car in reverse and drive. Not park and neutral...that will put no load on the mounts.
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how easy is it to replace the trans mounts

I just purchased a 2001 S500 with 78k miles and found the same issue. Did anyone come up with a solution? Thanks!

I added that video to the video thread

I'm probably not much help since I don't have the final answer yet, but I have been chasing a very slight idle vibration/roughness for a very long time as well. I can't figure out where it's coming from. The bulk of it when I bought the car was the ABC damper, but it's still not 100% right. I have had the car on a lift and felt/listened all over the place under as well as in the bay trying to figure out what isn't properly isolated/what's causing it. No luck so far. I've fixed several suspect areas, but it still seems like something is vibrating ever so slightly. I know it's not my mounts...the entire engine cradle is dead smooth...can't hear or feel vibrations traveling through the mounts at all. Same goes for the trans mount. Can really feel it in the fenders and upper radiator support. Can't tell you how much time I've spent tracing every little line to make sure they are all properly isolated, etc...still haven't solved the issue. The worst vibrations I have picked up on are in the oil cooler lines and oil cooler itself, but everything there appears to be well isolated like it should be.

It's been a pretty big mystery and if I ever figure it out I will definitely post up the solution. I suspect it could still be ABC/power steering related and may be a "normal" amount of vibration. It's pretty subtle, but I tend to get obsessed with little things like that.

I have never heard of "fake spark plugs" would it be worth counterfeiting a 7 dollar item??

To rule out any belt driven component, run the car for a minute with the drive belt removed and see if the issue persists. Keep in mind your alternator and water pump won't be spinning, so your battery won't be charging and coolant won't be flowing, do not run it like this for more than a minute or the battery will die and your engine will overheat. You will also likely generate a lot of codes in the ECU, so have a scanner ready to clear them.