Hello everyone,

I have a 2008 C300 4Matic that has a rough vibration at idle. I know there are a lot of forums on this topic, but I haven’t found one with my exact symptoms as of yet so I wanted to get some input.

It has a super rough vibration at idle, mainly when in drive. However, it does not happen every single time (only most), and it usually gets helped by being placed in neutral or park. Again however, it doesn’t help every time. Sometimes it makes no difference. I also noticed that occasionally when I shift into drive, the RPM’s will drop super low (400ish) even when it’s cold, but then will rise and be perfectly fine. Also, I do have a slight vibration sometimes when accelerating lightly (never hard acceleration), but it’s not super common so I don’t really believe that they’re related and the vibration is probably just the car deciding to shift or not.

Here’s the weird part though that I have never seen discussed on here. When I pop the hood and look for myself, ONLY the drivers side of the engine is vibration. There’s no jerking when shifting or raising the RPM’s, just only vibration. This leads me to believe that maybe the drivers side motor mount is bad and the rest are ok, but I’ve never dealt with motor mounts failing before so I’m not too familiar and don’t want to just throw parts at it.

Also worth noting, I do believe that I need a new belt tensioner and belt, so will be replacing those as well as the two idler pulleys soon. It squeaks only on super cold startups and I did notice there is a little more give in the belt than there should be. It has 177k miles and the belt looks original, as well as all the pulleys so no surprise there.

If anyone has any other ideas, or even could help me determine which mount has failed or another course of action, please let me know and thank you!

 

Inspect the engine mount for any fluid leaking from it. I believe they are filled with some type of hydraulic fluid. look all around with a flashlight.

Inspect the drive belt for wear or cracking. With my car the belt had looked like it was bouncing, looking closer the arm and pulley on the tensioner looked like it was pulsating up and down, like the belt was riding up it then falling. So i replaced the tensioner and the belt and it ran much smoother, ran perfectly. I replaced the upper idler pulley at that time also. i had waited a year or two to replace the lower idler pulley, there was really no signs to replace it i just wanted to as preventative maintenance. When i removed the old one I was surprised to see it was still running in great condition, bearing was very smooth and no sound spinning it by hand. I replaced it anyways but feel i really didnt need to.

Look on the tensioner itself for the markings that indicate how stretched the belt it. there should be two lines on it, and then another line on the arm part. All the way to the left line is a good belt, all the way to the right is stretched.

Heres an example of a good belt. happens to be mine.

 

Thank you so much, Tim! You’ve replied to a few of my forums now and I greatly appreciate it. You’re a very knowledgeable man. I plan on replacing the belt and tensioner due to the same reason you described, and the belt is a little dry-rotted and looks original so I want to do it preventatively. Same for both upper and lower idler pulley. I plan to also check all the other pulleys once the belt is off, and if I need to buy more than I will.

I am not afraid to spend the money if it keeps the car running great, but quick question for you. Did you opt for the genuine parts? Or did you use another brand? I found an INA kit with all 4 parts I mentioned, and supposedly they’re exactly identical to the genuine parts just without the logo.

As for the motor mounts, I’m by no means an expert as I’ve worked with a lot of things in the past, but never motor mounts. But I did shove my phone down there and get a video from every angle I could, and honestly they look great. They’re both a similar height, no cracking, no obvious leaking fluids or stains. The bolts don’t appear to be rusty or anything either, which makes me think they were replaced at some point, who knows when or what brand they were though.

Edit:
One last question, I apologize I didn’t think of this before. Would it be advisable to remove my belt, and then start the engine and see if that solves the vibration at idle? If it did, I could only assume that replacing what we discussed would solve my problem?

 

I would not run the engine without the belt very long. It drives the water pump, alternator, power steering pump, and A/C. Water pump and alternator would be my biggest concern. Also, don't forget the transmission mount.

 

Do you know how to test for bad engine mounts? Apply the foot brake, put the transmission in reverse, and observe the engine movement when giving it some gas. If there is significant engine rock, you have bad mounts. A little movement is normal; a lot is not. Here is another way to check.


Hint: You can do what this guy describes without a lift.

 

At around 3 minutes, you will see excessive engine movement.

 

I always recommend Genuine Mercedes parts, or from the manufacturer who makes that specific part since its usually less expensive. Usually the Mercedes parts will have a label or be stamped by the manufacturer that made it.

For the two idler pulleys I installed INA brand, which I believe makes the Mercedes pulleys.

Belt I installed Continental.

For the belt tensioner I installed Gates brand. The Genuine Mercedes tensioner is stamped Gates right on it. FCP also has a Febi and INA tensioner that is actually made by Gates also. So go with whatever is less expensive I say, as long as it is made by Gates.

I found is less expensive buying the parts separate.

Nothing wrong with removing the belt and running the engine shortly. Its recommended to when chasing down noises from the belt or pulleys.

 

I will definitely have to give removing the belt a try then! Also, I went out today and checked out that wear indicator, and my belt is a good inch further to the right than the right-most mark on there, long overdue for this! I’ll have to get to ordering those parts. Thank you

Edit:
Even better, after doing some more research it seems that kit I found on FCP that I mentioned is actually two INA idler pulleys, febi bilstein tensioner, and genuine MB belt! For only $170, I’ll definitely take that lol

 

I will have to check out that video and see this other method you describe, but as for the traditional method of accelerating and shifting gears and all that, there is no excessive movement in the engine. There is no jerking at all even, not even a little bit. It quite literally is just that the driver’s side half of my engine vibrates quite violently, and the other half is stable as can be. It isn’t a jerk either, it’s honestly just a very violent vibration, not enough to be called any other term.

That’s why I wanted to post on here, because all *typical* signs of motor mount failure aren’t there. There’s just a hard vibrations when stopped like I described. It’s honestly very strange. Given what Tim has been saying though, and my just general experience in the past, I’m beginning to think that my belt being as stretched as it is has been causing my engine to vibrate when it “catches” the belt on the tensioner again, and it gets worse when it’s in drive because the RPM’s drop slightly and it makes the catching take longer, hence making vibrations worse.

One other theory is that I did remove my MAF sensor to clean it up (I always do when I buy a used vehicle) and I noticed the throttle body was a little gummed up, but I only had roughly a quarter can of cleaner left over. I cleaned the front decently, but didn’t touch the back or the edges yet. The plan is to also buy a new oil separator and whatever else I need for the PCV system and replace that as well.

That’s just my theory, I am ordering the parts tomorrow and will definitely update you all if this fixes it, for anyone else that may run into this problem in the future.

 

If you don't already have a Mercedes-specific scanner, you need to spend the money on one. The LAUNCH Creader Elite 2.0 BENZ is generally recommended. You can find it on Amazon for about $160 or from KingBolen for $109. KingBolen does ship from within the US, though it may take a day or two longer than Amazon Prime.

 

I was actually looking at getting an iCarsoft CR Max enhanced for up to 2025 model years. Thoughts on that one? I’ve heard it’s just about the next best thing you can get sub 1k (besides some Chinese STAR copy).

 

I'll give you one that I had to learn myself. I had a mysterious vibration similar to what you're describing on my 3.5L M272, but wasn't localized to one side or the other.

At this point, I've done literally every single "common issue" on this engine to the point I'm doing things a 2nd and sometimes 3rd time for various reasons of bad luck or bad brand new parts. The one thing I didn't think I was going to need to do (at the mileage I did, roughly 115k mi) was injectors. I pulled my plugs and noticed a few stunk like fuel and were being fuel fouled.

As with most things on my car, the symptoms are in my face, but the data (in this case fuel trims, etc) were not outside of the acceptable range at all. However, replacing the injectors was the only thing that solved the vibration. Oddly enough, when I did belt, tensioner and pulleys upon 1st getting the car, I noticed zero difference even with noticeably shot pulleys, squeaking belt, and loose floppy tensioner. I also did engine/trans mounts (trans mount far worse off than engine mounts, engine mounts at 50% life compared to new), and the difference was possibly more placebo than reality. A difference was made though, just not what I was hoping for. Genuine and OE parts only for everything.

So far in the literal countless, endless list of jobs I've done on this car, the injectors made possibly the biggest difference in engine smoothness. My spark plugs are currently carboned up partly due to those injectors and from what I suppose is my 4 yr old pcv system again giving up the ghost, yet the injectors still gave me the idle I thought new plugs would give.

 

I used to be a big fan of iCarsoft and have both the iCarsoft MBII and iCarsoft MB V3, and they don't come close to the LAUNCH in capability. I can't speak for the CR Max.

For serious diagnostics, I go to my Start Diagnosis System (SDS) XENTRY. But for 90% of everything else, I go to my LAUNCH. By the way, the LAUNCH also has the ability to add additional brands. Do you have more than one brand of car?

 

Update: Grabbed a video of my belt when switching into drive from park, and then back to park at the end. Definitely leaning towards that being my vibration now lol, it’s pretty violent. I’ll attach it here.

 

This was another idea I had, thank you! I actually have some Techron to throw in the tank as soon as it gets low enough lol. I grabbed a video of my belt when switching into drive from park, and then back to park at the end. Definitely leaning towards that being my vibration now lol, it’s pretty violent. I’ll attach it here for you.

And thank you for that idea, if this doesn’t work and the Techron makes any difference at all, this will be my next step.

 

My fiancee has a toyota, and I do periodic work on all kinds of different cars. I’m the family mechanic, and my fiancee has a very large family lol

 

No problem man! Even if just throwing **** to the wind, my chase of idle vibration ended with the injectors which is still bizarre to me given how "mechanical" the vibration felt. My fuel trims were also rich enough to justify, especially compared to how nice they looked after things like fitting fresh upstream lambda's.

That clip was extremely violent! Looked like the belt was about to jump off the idlers and tensioner pulleys. No other vibrations felt throughout the chassis/driveline? Well, by the looks of that hopefully you do get away with the easy and satisfying job of replacing those. Are they original? They look so.

 

I just bought the vehicle a little over a week ago, but I believe they are original based on how stretched the belt is. It’s well past the built-in wear indicator. And nope, only when put in drive.

I will admit that I wouldn’t be surprised if I do end up needing to swap a motor mount or the transmission mount, but I do believe that switching out the belt will help a LOT, and the pulleys and tensioner I’m just going to do anyways in case they’re past their replacement date. There’s no way to know until I swap them out though, and the parts get here Tuesday so I will definitely update this forum on if it helps/solves the problem.

 

I really like Techron. In my opinion from personal experience and watching videos on using Techron I think it does a good job of cleaning things. Recently my engine felt like it was running a little rough, at stop lights I could feel my pants vibrating on my legs. I am on my 2nd 12oz bottle of Techron and the engine seems to be running perfectly now.

Not sure if it was mentioned but I would service the transmission, replace the filter and fluid. I dont think draining the torque converter is absolutely necessary unless the old fluid looks really bad. I recently service my transmission more so to replace a severely rusted transmission pan and the fluid looked like normally used fluid so I didn't bother with the TC. I still have a few quarts of new fluid that I had planned on using, just remove the drain plug and drain whatever comes out then pump in new fluid so its full.

The belt does look like its stretched and slapping around. I would not be surprised if replacing the belt will solve your vibration issue.

For the PCV system you can reach back at the passenger side breather cover and pop off the hose, look inside to see how much oil is in it. There should be a little bit but not flooded with oil. use something like a q-tip and rub it around inside. I replaced my passenger side cover and the drip pan in there but turned out they were in really good condition and i didnt even need to. But that gasket is pretty much one time use so once the covers removed you need a new one and they dont sell the gasket separately. For the drivers side cover there really is no need to replace it unless oil is leaking out from the gasket, there are no moving parts inside to wear. I removed mine, cleaned it and put it back on. That gasket is more like a hard rubber so was able to re-use. But i wouldnt touch it unless its leaking, someone on here had a hard time reinstalling new covers and they wouldnt stop leaking for some reason. While you are at if you want disconnect the hose at that cover and where it goes onto the throttle body and spray some compressed air thru it just to blow out any oil that may be inside. I used air from a can. I believe this same hose also connects to the purge valve.

Heres my thread on what the drivers cover looks like inside: https://mbworld.org/forums/c-class-w...ks-photos.html

Heres one where I show what my passenger side cover and drip pan looked like. They looked good to me. https://mbworld.org/forums/c-class-w...does-work.html

 

I also am a huge fan of Techron. It saved me from some major idling issues and lean codes in my old Yukon XL. I just put in 20oz with this tank and so far am 50 miles in, but I’m only running it preventatively this time thankfully.

As for the PCV system, I did read that there’s essentially no point in replacing the drivers side cover unless it’s leaking as well. I am currently leaning towards replacing the cover and drip pan, as well as both hoses from passenger and driver side cover as they’re only $20 total from FCP. My cam plug on my passenger side was leaking and I just replaced so I’m not too sure on if the cover is leaking or not, figure for $100 I can’t go wrong with replacing the cover and drip pan, even if it is only preventative.

For the transmission, I do plan on doing a fluid and filter change at some point. I do have the 4Matic, and I’ve read that the TC is pretty much the only flaw with the system so I probably will drain it to be safe. Either that or I’ll just do two fluid changes a few weeks apart and then continue with every 30k miles from then on.

I already changed both differential fluids and put in some fresh fluid, as well as new tires, pads and rotors, plugs and coils (as well as a few bolts that were starting to rust), front axles, and some more things that are slipping my mind. All were replaced with Genuine Mercedes parts as well, I found a dealer down in Arizona that shops nationwide so I’v been using them whenever FCP doesn’t carry what I need. I try to always replace anything that may have missed its scheduled maintenance whenever I buy a new vehicle. That’s why I am leaning towards replacing the PCV and everything as well as who knows when/if it was ever done.

I have been thoroughly enjoying the car though and am very thankful that besides the axles, cam plug, and tires, everything has just been preventative so far and nothing has been broken. I bought the car with all 3 of those being disclosed by previous owner though which is a good sign. I hope the ownership experience stays this way for sure.

 

Then I strongly recommend the LAUNCH Creader Elite 2.0 BENZ as mentioned above. You can add different brands as needed.

 

Great, thank you!

 

If i didnt already have one I would get a Launch scanner. They are on AliExpress for less than $90.

 

Ah ok, so perhaps it's safe to assume your engine & trans mounts are original as well? And by additional chassis/driveline vibrations, I did mean do you also feel those when shifting into D. I also look after my sister's W203 ('07 C230 w/ 2.5L M272) and her engine/trans mounts are dead. She works from home so the car sits a lot & our Aunt was the 1st owner, so I know the car's entire life & history. Her drivetrain literally leaps when going from P to D, R to D, etc but mainly when cold/1st start oddly enough. It's fairly stable otherwise, especially considering I've had eyes on them and know their sad state. It feels as violent as your video looked, but like magic, my sister is unbothered by it or magically doesn't notice hahaha....still they will be addressed.

No need to wait until parts arrive for a conclusion to pulleys. Are you familiar with the hole in the belt tensioner made for the little tool to slide in? You can place the tensioner in "service mode" (necessary to remove tensioner as one of 2 bolts is hidden by the tensioner pulley) then once in service mode, you can slip the belt off all 3 troublesome pulleys (while still leaving it in place on all other belt driven components) and spin them. It will be no mystery if dead. The bearings will scream at you and they will not spin smoothly as they should.

My og dead and dripping tensioner. 17mm w/ breaker bar goes on stud (left) to relieve tension. New tensioner comes w/ a small punch pre installed in circled hole. This holds tensioner in place so you can do your thing

Left, small punch. Very handy to keep around. Also about the same size as a #4 Allen which will also work or anything similar

 

My og half dead engine mount(s). New ones had a gap about twice as big as what's circled here and had about 50% less play via large screwdriver placed in top bolt hole.

OE Reference number

New trans mount waiting for sister's W203, same for my 2010 C207 E350 and most all W204's as they are the same platform plus or minus 2 or 4 doors. Circled area will have a finger sized gap on a dead mount

I don't think your profile has your car's details? You may have mentioned 4Matic? Apologies if so, these mounts I believe are RWD specific.