Questions about a noise in my w114 - PeachParts Mercedes-Benz Forum

wolf_walker · #1 01-24-2009, 01:44 AM

73 280.

It's had this since I had it, and I know it's hard to describe a noise, but I figured I'd give it a shot.

It's a hiss, almost like a really fast rattle that's so high in frequency that it's a hiss.

It isn't road speed related.
It isn't RPM related.
It seems to be related to engine load, or a combo of engine load and rpm or road speed.
It's not there or I can't hear it at WOT.
It's not there at all on decel.
I"m 99% sure it's not a vacuum leak, I've heard my share of them.

It tends to ramp up till the shift point and then goes away, and then comes back for the next gear. It's most prominent in 4th at 40mph. I can actually induce the noise and make it go away with the throttle by pushing in the accelerator.

I think it could be the exhaust, this twin small tube exhaust setup seems to make some odd noises at times. I plan to try and check for leaks this weekend if the weather holds, but I honestly wonder if it's not some sort of resonance with the pipe. I'm not sure how to test for that other than maybe using large zipties to hold pressure on the exhaust pipes here and there, till they melt anyway.

My other two thoughts are the trans, somehow, or the center driveshaft bearing. The latter being the weakest theory, I checked play in the CSB and looked over the guibo's and they all look fine to me. The trans, I don't know, I guess it's possible but it's beyond my knowledge of how these units are built internally to think of what could cause this sound.

It's not mild though, it's quite audible, and I know it's audible outside the car too going through small tunnels and such. I can't reproduce it without the car moving, and I'd be willing to be with the drive wheels off the ground it'll not do it either. So, other than poking at the exhaust I'm pretty stumped.
Has anyone heard anything like this or does anything ring a bell at all?
Thanks..

mercmad6.3 · #2 01-24-2009, 03:21 AM

Quote:

My other two thoughts are the trans, somehow, or the center driveshaft bearing. The latter being the weakest theory, I checked play in the CSB and looked over the guibo's and they all look fine to me. The trans, I don't know, I guess it's possible but it's beyond my knowledge of how these units are built internally to think of what could cause this sound.

I would say it is the center bearing .The only way to check it is to undo the bracket so you can lower it and rotate it with your fingers. Only this way can you feel any wear spots in the bearing itself.
While you have the car in the air check underneath for any rubber floor bungs that may be missing and the rear wheel bearings.
Have you checked the diff oil level.

wolf_walker · #3 01-24-2009, 12:32 PM

Have not checked the diff oil as I can't get the plug out, even with my 4ft breaker bar. It's possible I guess but there isn't a ton play or such.
I'll see about drooping the CSB shortly.

wolf_walker · #4 01-24-2009, 09:45 PM

It's defanantly center-ish of the car. And holding 3rd manually while accelerating very slowly, as I tend to do when the car is cold since, well, it's a solex carb still, it will make the noise starting at 30 or so and as rpm and speed climb with no movement of the throttle the noise goes away, like it's a resonance somehow. Got to be exhaust or the CSB, and I've never heard a CSB make noise like this.

Mike D · #5 01-25-2009, 07:22 AM

The '73 M110 has the "mickey mouse" EGR set-up. Have you checked the EGR tube connections at the exhaust and intake manifold?

wolf_walker · #6 01-25-2009, 02:33 PM

Yep, it's removed, I was hoping that was it too.

mercmad6.3 · #7 01-25-2009, 08:46 PM

Quote:

Originally Posted by wolf_walker

It's defanantly center-ish of the car. And holding 3rd manually while accelerating very slowly, as I tend to do when the car is cold since, well, it's a solex carb still, it will make the noise starting at 30 or so and as rpm and speed climb with no movement of the throttle the noise goes away, like it's a resonance somehow. Got to be exhaust or the CSB, and I've never heard a CSB make noise like this.

With the car idling,just stick your hand over the end of the exhaust and listen,if it's leaking enough to make a hissing noise it will chuff through the hole. I've never heard any exhaust leakage make a sound like that though,they make a noise throughout the rev range not just in any specific gear..

wolf_walker · #8 01-25-2009, 08:58 PM

Quote:

Originally Posted by mercmad6.3

With the car idling,just stick your hand over the end of the exhaust and listen,if it's leaking enough to make a hissing noise it will chuff through the hole. I've never heard any exhaust leakage make a sound like that though,they make a noise throughout the rev range not just in any specific gear..

I agree, and plan on that when the weather cooperates. I've also never had a car with two tiny exhaust pipes the length of the car too though. And I'd be willing to bet there is some support or hanger that is missing from past service, seems like there always is. The exhaust looks pretty recent though.
I can rap the throttle in park and just as the rpm's start to come down I can hear something make a rattle/buzz for a split second.

Life would be so much easier if I had a chassis dyno..

mercmad6.3 · #9 01-25-2009, 09:11 PM

That rattle sounds to me like the noise from a worn timing chain. The twin pipes are there because it would require a huge pipe to flow the same amount of gas . The exhaust should be reasonably secure but not tight as it designed to move as it heats and expands. Give it a good rattle by shaking the tail pipe. If a baffle is loose inside you may hear it .

wolf_walker · #10 01-25-2009, 10:02 PM

Chain stretch was good, I thought that too and actually bought a tensioner, but the quick stretch check was good and it's quiet on cold start and all that. Car only has 118K on it. It sounds like a damn rattlesnake if you've ever heard one, it has to be a resonance being that it'll pass through it at a steady acceleration I'd think.

mercmad6.3 · #11 01-25-2009, 10:42 PM

Quote:

Originally Posted by wolf_walker

Cha. It sounds like a damn rattlesnake if you've ever heard one, it has to be a resonance being that it'll pass through it at a steady acceleration I'd think.

No rattlesnakes here, only Killers.... I did think of suggesting that look for an animal though

hissing and rattling ,could be a snake,not an OZ one though,thety are quiet and just bite and kill you .
you could try this Critter repellent

wolf_walker · #12 01-25-2009, 10:55 PM

The assorted wildlife in Australia has always provoked a lot of respect for the people that wonder around in it. I detest stuff that can kill me without announcing it's presence and giving me a fair opportunity to shoot it.

wolf_walker · #13 01-31-2009, 12:57 PM

Well I looked over the exhaust and did find one small leak a a coupling, but it's not the source of the noise.

I swear I keep coming back to it being the trans. If it was a GM trans I'd wonder if there wasn't something loose on that rotating shaft dealy with the centifugal weights I forget the name of. I might change the fluid again and see if it effects it I guess.

And speaking of the exhaust, have you ever noticed the forward twin pipes are loud in the way that a header is? And the volume and pressure of the exhaust exiting the muffler sure sounds like it's too small. It'd be interesting to run either a larger single or continue the twin pipes all the way to the rear into a larger dual in/out muffler.
I wonder if the stainless steel does not have a role in the sound as well. I've heard other aftermarket stainless parts be uniquely noisy before.

mercmad6.3 · #14 01-31-2009, 05:00 PM

just a quick note on the exhaust. The idea of the twin pipes is to allow a large volume of gas through. if you were to replace it with a single pipe,the equivalent size is close to 5" diameter. Stainless pipes do resonate badly on any car. OK for long life but they are not much good if like to tune the exhaust sound for what ever reason.
I still think you should spend an hour or so dropping the shaft and checking the bearing and the diff oil level. If the filler plug won't come out remove the cover and replace it with another you can loosen the plug on.

If it were an earlier model trans I would suggest the governor right angle drive but on Mercedes trans such noises don't come in until after 45 mph and stop at 60 mph.

wolf_walker · #15 01-31-2009, 07:22 PM

The center bearing is on my list, I usually on older cars just replace the guibo(s) and bearing and such as a matter of course, though these show no real wear and there is no perceptible up and down play in the center bearing.
It could well make noise spinning, but I doubt it. It just don't feel like that kind of noise. I can't induce it under any circumstances in 4th above 45mph, no matter what the load. It's quite odd.
The fact that it comes and goes right before shift points at differing road speeds, just don't seem like driveshaft. But I have a feeling I'll end up replacing it trying to figure this one out. Damn I wish I had a chassis dyno.

I generally trust Mercedes engineering, but I'm not seeing it here. Unless they were trying to create some low end torque to offset the M110's natural tendency's using exhaust velocity maybe. Single 2.5" with good bends would be better suited otherwise it looks like to me. A 130hp 2.8 does not need a 5" exhaust, it would in fact hurt power. The little crossover between the two headpipes and the fact they bothered running two makes me wonder what they were trying to accomplish though. It just doesn't feel right.
And exhaust tech has come a long way.