I Can't Believe How Smooth My Idle Is Now!!!

[gmercoleza]

1991 300E, 126,000 miles. Up until last night, the car had a rough idle. I mean REALLY rough. Stopped at a light with the trans in D, it would shake so bad that the hood would visibly jump around. The dash and something under the steering column would also rattle and occasionally emit this very loud, deep groan, sort of like a harmonic vibration that you could not stop. I thought something was loose under there. Sun visors and doors also produced rattling and rubbing noises. It was somewhat embarrassing whenever I had passengers.

I had been perplexed since the cap, rotor, wires, and plugs were all replaced about 3 months ago and the car passed the state emissions test with flying colors. I knew the engine wasn't misfiring or running poorly in any way. I figured the car was just really old and that things were just rattled loose from the 13 years and 126,000 miles of driving. In short, I began to just accept the rough idling as part of the character of the car. I recalled my last Mercedes, a 1986 560SEL, idled somewhat rough as well (although not quite as bad), so I reasoned that these German cars must just end up a little less refined in some way once they were past their prime.

I had been doing some research regarding the rough idle, and stumbled upon a few threads that talked about motor mounts. I saw a pair of them at a good price on eBay, so I decided to take a chance. Let me tell you, this is the biggest bang for the buck improvement I have ever done to any of the cars I have ever owned, hands down.

Most of the threads here indicated that only the passenger side mount usually needs to be replaced, so I only replaced that one. It was very easy and only took me about 30 minutes. Now, stopped at a light with the trans in D, it is smooth as glass. You can barely even tell the engine is running. Simply incredible!

If you have been struggling with a rough idle on your W124, stop what you are doing now and GO GET A MOTOR MOUNT! You won't regret it. Instead, you will be kicking yourself for not having done it sooner. In fact, if anybody here wants one, I will sell my other motor mount for $40 shipped. I paid $70 on eBay for the pair, but most likely you don't need two. The passenger side mount fails prematurely due to the heat from the exhaust manifold which is only a couple inches away. Mine had collapsed about 3/4 inch compared to the new one (I will post pics soon).

If anyone wants the motor mount for $40 shipped, please let me know. It will also work on a 201.

[mctwin2kman]

Those motor mounts were revised in the 90's I believe. Which was when they made them a little bit taller. Not much, but enough. You should always replace both when doing them. So that the engine will sit properly. You could end up with the engine slightly angled more than it should be and could cause premature failure of the tranny mount or that newly replaced motor mount. Just an FYI!

[pentoman]

Sounds nice..

is it smoother throughout the rev range?

I have a problem not with a rough idle, but instead with vibrations at ~3500 rpm and general Noise, Vibration and Harshness at high revs/high speed cruising.

Russ

[BenzRepair]

Had a horrible vibration at idle after a rebuild; I'm always quick to assume that it's something I did, but once I settled down and replaced the motor mounts, the customer complemented me and commented it never drove that smooth, ever.

[gmercoleza]

Originally Posted by pentoman

Sounds nice..

is it smoother throughout the rev range?

I have a problem not with a rough idle, but instead with vibrations at ~3500 rpm and general Noise, Vibration and Harshness at high revs/high speed cruising.

Russ

Now that I have replaced the failed mount, I realize that the car was generally "noisier" overall with the failed mount. For instance, the very next day after replacing the mount, when jumping aggressively on the throttle in order to pass a semi, I noticed a slight dieseling noise. After a few minutes of playing around and experimenting with the throttle, I realized this was actually engine pinging from the 87 octane I have been running for the last month. I had gradually been working my way down from 91 octane, all the while listening intently for pinging, which I never heard. So I settled on running 87 octane. Of course, now that the motor mount has been replaced and I can actually hear pinging, I will be working my way back up and may eventually have to start running premium again.

Also, since the mount has been replaced, I now notice peculiar squeaks and other noises from the suspension. In particular, I notice a noise somewhere in the rear which resembles a somewhat metallic squeak. It is intermittent and only seems to occur when the car shifts weight laterally, such as when cornering on a left turn and then suddenly shifting to a right turn and vice versa. I'm not quite certain what the noise is; I just had the car aligned a few weeks ago and the tech did not mention that any suspension parts were worn out or needed attention. It sounds sort of like a loose bushing, but I can't be sure. With the time off over the holidays, I will have to jack the car up and take a look to make sure everything is in order.

So it would appear that, even though my car seemed to idle and run smoothly at all times other than with the trans in D while stopped, in actuality it was running more roughly than I was able to discern.

But in specific answer to your question, although I did not necessarily experience any harmonic vibrations at ~3500 RPM, I do believe the new motor mount has smoothed the engine out considerably throughout the entire RPM range. In general, all noise, vibrations, and harshness have been greatly minimized. The car doesn't "jerk" at startup anymore (it feels kinda weird and silent now), and - unfortunately - there is noticeably less feedback in the form of noise and vibration from the engine, to the point that it seems to have transformed the car's driving dynamics from a somewhat sporty, throaty response to a muted, hushed, refined tone.

I would say it's worth a shot if your motor mounts haven't been replaced in a while. You may end up fixing problems you didn't know you had.

If you want my other unused motor mount (or anyone else for that matter) you can have it for $40 shipped. Otherwise, I will be keeping it as a spare.

[pentoman]

Thanks for taking the time to write a reply gmercoleza.. much appreciated . I just took a ride as a passenger in the 190E, and think there is too much NVH.. so engine mounts are on the list .

Russ

[gmercoleza]

Originally Posted by mctwin2kman

Those motor mounts were revised in the 90's I believe. Which was when they made them a little bit taller. Not much, but enough. You should always replace both when doing them. So that the engine will sit properly. You could end up with the engine slightly angled more than it should be and could cause premature failure of the tranny mount or that newly replaced motor mount. Just an FYI!

I've been thinking about what you said and thank you for the tip. However, I've been analyzing the possibilities and have decided that there really isn't any risk of any under-hood damage here, at least not great enough for me to go through the trouble of installing the other mount. I'm surely no geometry expert, but it would seem to me that the effect of the 3/4 inch height difference is minimized by both the distance between the 2 mounts (about 1.5 to 2 feet) and also by the fact that the original mount was surely at least 1/2 inch taller in its original form (you can tell by looking at the collapsed portion). Therefore, even if the new mount is a redesign and is substantially taller than the original uncollapsed mount, we are still talking about only 1 or 2 degrees of engine angle. This is further minimized by the clockwise (from the front) rotation of the crankshaft, which exerts downwards force on the passenger side mount (compressing it somewhat) while at the same time "pulling" on the driver side mount (stretching it somewhat), effectively counter-rotating the engine a slight - but measurable - amount, and equalizing all the angles. I'm also looking at this from the perspective that the engine/trans assembly are ALWAYS in motion while the car is being driven, in other words there is a great deal of stretching, pushing, and pulling going on under the hood, especially when you take into account driving aggressively, stomping on the gas while passing, hitting potholes, braking hard, cornering, etc. With the flexible nature of the mounts and all the great forces moving that whole assembly, I am quite certain 1 or 2 degrees of engine angle deviation won't make any measurable difference or put any substantial additional wear on either of the other 2 mounts.

But thanks for the thought and suggestion!

[dirtysocks]

gmercoleza:What did you use to hold the motor up while changing the mount? Did you use a spring jack? I desperately need to change mine, but I only have access to a hydraulic jack, which I know continually go down as they support weight (well mine does anyway). This does happen quite slowly on my jack, but I am just nervous about the engine falling to far and damaging something. How long would it take me to replace BOTH the mounts? (with the car up on ramps and limited movement). Is it easier to approach them from the top, i.e. under hood? Anyone else have a suggestion for making this project easier for someone with a hydraulic jack?

Thanks for your time.

[gmercoleza]

Originally Posted by dirtysocks

gmercoleza:What did you use to hold the motor up while changing the mount? Did you use a spring jack? I desperately need to change mine, but I only have access to a hydraulic jack, which I know continually go down as they support weight (well mine does anyway). This does happen quite slowly on my jack, but I am just nervous about the engine falling to far and damaging something. How long would it take me to replace BOTH the mounts? (with the car up on ramps and limited movement). Is it easier to approach them from the top, i.e. under hood? Anyone else have a suggestion for making this project easier for someone with a hydraulic jack?

Thanks for your time.

I can only tell you it took me about 20 or 30 minutes to do the passenger side mount. I can't comment on the driver's side, but from what I've read that one can take you an hour by itself because it's quite hard to get to. Personally, I don't think replacing the driver's side is necessary (keep reading to find out why I think this way). You will need to get at the mount both from under the hood and from underneath the car, but trust me - it's quite easy.

I used a hydraulic jack from underneath, with a 2 x 4 used to spread the load evenly across the front of the oilpan (I jacked at the front to minimize fulcrum effect). I know the debate over whether or not to use a piece of wood rages on, but my jack has an irregular shaped cradle which has 4 raised turrets. Rather than allow force to be concentrated on 1 or 2 of these turrets, I used a piece of wood.

If your jack lowers slowly, there is a problem. It shouldn't leak at all, if you have the valve mechanism properly and completely closed. If the valve is closed tight all the way, and the jack still lowers (even if very slowly), there is a leak somewhere in the hydraulic system and the jack is unsafe. That leak could suddenly give way, injuring or killing you. Discard of it immediately; nothing's worth risking death or injury.

With that said, there is NO DANGER AT ALL of the engine "falling through" so to speak. Pop your hood and you'll see what I mean - the engine mounting arm actually sits on top of the motor mount. If your jack gives way, the engine just falls back on the mount. If the mount is removed and the jack gives way, the engine just falls on the crossmember below.

I won't be held responsible for anyone's accidents (DISCLAIMER!), but in my opinion you could just as easily use a scissor jack or a bottle jack - you are only lifting the engine for a minute or less, just enough to slip the old mount out and swap in the new mount. And you're not lifting much weight - even though I used a jack on mine, I was still able to "lift" or nudge the engine up another few millimeters just using my bare hand (so you may be able to use your leaky jack and supplement the lifting force with your own hands). Of course, this means that you remove the top and bottom mounting bolts before performing the swap. You will need a 17 mm combo wrench on the top bolt, and a hex key on the bottom bolt (I don't remember what size, but it might be 3/16). My bolts were incredibly loose - I barely used any pressure to remove them (YIKES!). I wonder how they stayed in there like that. Anyway, when replacing the bolts I didn't torque or anything, just tightened as much as I could humanly do (which was way tighter than they were to begin with). For peace of mind, I used a hammer on the 17 mm combo wrench to tighten the top bolt.

You'll first need to set the parking brake, then chock the rear wheels and jack the car up enough for you to crawl under and loosen that hex bolt - for safety's sake, be sure to use a jack stand while you're under there. Or you mentioned putting it up on ramps - that's probably the best option. I didn't use ramps because my hood would hit the ceiling of my garage if extended in the fully upright position. But ramps would definitely provide the most working room.

See my above description of the torque effect the clockwise-rotating crankshaft has on the mounts and you will understand why the idle is especially rough with the trans in D. What happens is the passenger side mount gets further compressed, and if it's actually collapsed like mine was, with all the vibration damping oil leaked out, the metal housing will actually make contact with the cross member below. The result is that vibrations are transmitted directly from the engine through the metal motor mount housing and onto the crossmember (the chassis), and you end up feeling the vibrations clearly in your seat and on the floor, etc. With that said, replacing just the passenger side mount will clear up the problem - I'm fairly confident you won't need a pair of motor mounts, just one. The driver's side mount gets "pulled" rather than pushed.

Again, if anyone wants, I will sell my spare motor mount for $40 shipped to the 48 states - PayPal only. Or you can order from Fastlane; I think they were $60 + shipping the last time I checked. I am told that Fastlane is a great way to get parts (though I haven't used it yet), and the prices really are great (check with your dealer or local parts supplier first if you don't believe me).