All M104 engine owners, Please Read
 

My 1995 C280 with M104 motor seems loud at idle. Now I know that this engine is inherently not the most quiet engine when idling, but I want a feel from the other members.
When in Park or Neutral, idle is much smoother, but when in Drive, and stopped in idling, it sounds like the engine is rattling. I think that I'm just over-reacting, but I'd just like some reassurance.
BTW, my engine has 104k miles on it now, and I am pretty sure the idle wasn't this loud when I got it at 68k miles. (Saw a tech once, and he said that it definitly wasn't the Timing Chain, and that the engine sounded fine.)

What do you think your M104 engine sounds like at idle?
Please provide mileage also. Thanks a bunch guys.

I think you will find your problem to be the belt tensioner
shock bushing.
Get it to do it [ 2 people, one to hold the brake in drive at idle] and pry on the shock with a large screwdriver. If the rattle stops, you have found it.

It will drive one Nuts.....

I had a problem with my 104 engine and it was the tensioner. After finally finding what it was, I could look at the top bushing of the tensioner and see the bushing was almost completely gone. Mine sounded almost like ignition "knock". After replacing, the engine is quiet as it can be.

Many have been diagnosed as timing chain because it does sound like that.
When you hear it, look real close at the top bushing and you may notice the shock moving back and forth on the mount .
[ sideways] . And as you have noticed, it does this mostly at idle , in gear.
. The side of the shock eye is hitting the washer on the mounting bolt. If this is the problem, it can be fixed by
inserting a thin rubber washer/o-ring between the washer and shock.
But if the wear is excessive, replace the shock.

My M104 idles quietly, but one time I did notice a rattling sound and the tech pinpointed the belt tensioner as the source of the noise.

Replaced it under warranty at 85K.

It's been quiet since then.

WHere is the belt tensioner...?
I'll take a look and see if I can find it.
I really hope this would make my idle quieter (still very smooth).

Do you think I'd be able to replace the tensioner myself?

A bit tight but you should do OK replacing it. On my car (93 300E) you almost have to remove the fan and that can be slightly difficult. On my car, there was a tensioner and a separate shock for it. I think on the newer 104s, it is together in one unit. It is located at the front of the engine and is one of the pulleys that the belt goes around not too far down from the top (just enough to make it interesting changing!). I think all of the other pulleys drive something but this is just to give the belt tension so you can probably pick it out without too much of a problem. This is one thing that is in the MB CDROM for the 124 series and I assume it about the same for your car. If you need it, I can probably e-mail you that sheet out of the CDROM.

I would really appreciate it. I am going to try and change this myself... save some money and hopefully learn a little bit!

What does the tensioner cost? Thanks!

Before you go changing the tensioner, reread the post.
The simple problem can be [ and most of the time is }
the Shock Bushing.
Much easier to change than the tensioner.
And less $$$.

Look 2" to the left and 3" down from the thermostat
housing. You will see a bolt that goes through the eye=
let of the shock and into the head. That is where the top bushing is and if you put a little pressure on it when it is rattling, the rattle may stop.
If it does , simply change this shock. Not the tensioner.

Dave,
There is a possibility that you have the newer modified tensioner.
I did some part research after reading Pfphipps post, and he is correct that there was a late124 chassis , 104 eng tensioner change. [ in '95 sometime].
According to partsshop Fast Lane, this is also the case with the C280. If your eng # is 12-634967 or later, the new part is $54.44
If prior, part is $118.94. This one list the seperate shock I mentioned and that is $22.
Nice to see a modified part that not only gets rid of a problem, but is half the price.
I have no pictures of the newer part in my books, so I don't know what the actual changes made are.

I went out to the garage to try to take a photo of the tensioner on my '95 E320 and there is not any way to get a shot of it without disassembling alot of stuff. If you have ever seen the serpentine belt tensioner on one of the newer Ford truck V8's then that is about what it looks like.

Little picture of the two styles. I have neverhad one of the new style fail on the 104 motor. But the similar one on early V8s fails.

My car was built 8/94... so I assumed it would be the older tensioner, which the MB dealer confirmed.

My engine number is: 104.941 12 031 966.

I hope that it's the shock only. pfphipps e-mailed me the diagrams regarding this for the M104 motor. I looked under the hood, but I just don't have a clue... in about an hour I'm going to take it next door to our independent mechanic and have him show me, hopefully with the assitance of the diagrams I printed (from pfphipps).

I'll report the results back later tonight.
Thanks again Pfphipps, and everyone else.

IT IS THE SHOCK!
 

Well tonight after I came hope, I opened up the hood, and disassembled the fan shroud and front engine cover.
I had all my papers (diagrams from pfphipps, printout of this post) layed out on the garage floor... I couldn't find the SHOCK or tensioner or anything.

Luckily, I remembered there was a diagram of the Poly-V-Belt in my owner's manual... so I checked out the tensioner with the engine running, in D, with parking brake fully engaged. The tensioner didn't seem to be moving.

I then proceeded to look once again 2 inches over and 3 inches down from the thermostat housing... but just couldn't find the "shock" I was imagining. Well minutes later with the flashlight, I saw what appeared to be an actual SHOCK. DUH!
I then did Arthur's recommended test, put the car in D with parking brake on, used a lug nut wrench and pryed against the shock... NO RATTLING!!

I then checked to see if the belt was slipping or had slipped in the past just to be sure. There were 0 stored belt slips and 0 current slips (tested via Climate Control info). Good to go.

Unfortunately, I was unable to detect any movement with the shock... which means I must replace the shock? Now I only need to change the shock. I did see the top screw for the shock, I think I can handle that ok... but I could not see where the bottom of the shock went to... how would I access this? I'm thinking I might have to remove the belly pan?

I'm buying the part tomorrow from the dealer (but tonight I'll buy my door vacuum actuator from Mercedesshop, not as... 'urgent').

I'm very excited. This will be the first engine-related work (done a millionn interior things) I'll have done on the car (besides changing oil and filter)! I'm thrilled!! ;)

I really appreciate everyone's help, especially pfphipps and arthur dalton, I couldn't have done it without you guys. THANKS AGAIN everyone!! :D :D :D


**2 hours later**
I just spoke with a friend of mine online who used to be a technician, and he said that shop time for changing the shock is 1 to 1 1/2 hours... that I would need a special tool and that I would need to remove the fan...... I'm not so excited anymore... :mad: :rolleyes: :eek: :confused: Basically, he recommended that I take it in.... but I could still try.
(A challenge would probably be good for me anyway......!)

Well David,
Welcome to the club of DIYers.
Now you are hooked and will have to buy every tool you see 'til you die !!!!

Anyway, there is an easy fix if the shock is rattling back and forth on the top bushing pin. [ which is common].
You can take the one lone mounting bolt that goes through the
the shock top eyelet and sandwich an "O" ring between the
shock eye and the washer. That takes up the side play [which
is the Rattle] dimension of the shock. The bushing itself
is still good. it is just sloppy on the side and this shimming works fine and last long. [And gets rid of the Rattle !! }
Of course, the best way is replace the shock, but the bottom mount is difficult.

Good work...