A weird murmur at exactly 10 on the tach 84 300SD

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Mia - 25 Sep 2005 02:20 GMT

Hi,

This has got to be the runner up in the "weird things that happen to my
old Benz" contest.

I have recently noticed this weird sound coming from the rear of my '84
300SD. At first I thought it was an empty gas can shuffling around in
the trunk, but then I looked at the tach and realized that the quiet,
yet deep and bassy sound coming from the back happened only when the
tachometer showed 10. Not 9.5, and not 11, but precisely 10.

I could make the sound appear and disappear by gently playing with the
gas pedal. It's unsettling.

Thanks!

Mia

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Jerry Wolfram - 25 Sep 2005 05:18 GMT

Mia -

Your tachometer has found the resonant frequency of something loose on
your car. Try tightening anything you find loose in the general
vicinity of the apparent noise source. If you're a true scientist
you'll perform this one at a time, then go test drive. When your tach
suddenly shows *other* than 10 when the noise appears, then the last
thing you tightened was the noise source (or something close to it).
Good luck in your quest . . .

Jerry Wolfram
'78 450 SL - 214K miles

> Hi,
>
[quoted text clipped - 13 lines]
>
> Mia

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Mia - 25 Sep 2005 07:04 GMT

Jerry,

I briefly looked around for things to tighten, nothing yet. I did find
out that when idling, the air filter casing shakes like mad, even
though the engine is barely vibrating. You know by any chance how to
tighten the air filter casing?

Thanks!

Mia

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T.G. Lambach - 25 Sep 2005 08:50 GMT

Harmonic vibration from the rear - check the exhaust pipe hangers -
rubber donuts that support the pipe and muffler. Also check that the
spare is tightened down in its well and that the pressboard cover over
the well isn't rattling.

Air cleaner. That's a sore subject on this motor. In 109K miles my car
has broken three support brackets - always in the same place - the rear
leg. It just can't take the sideways oscillation of the big air cleaner.
I had a reinforcement piece welded to the last new one. Then I added a
piece of heater hose as a bumper between the air cleaner and the valve
cover to snub the oscillations. If the reinforced bracket breaks I'm
going to mount the air cleaner on the body rather than the engine.

The air cleaner can is mounted on three rubber mounts. These break,
usually the rear one sometimes the outer (right side) one. They aren't
cheap, for what they are, but there's not much choice but to buy some
and replace them as needed, not sooner.

Ready for the cold weather? Got those valves adjusted yet?

Tom

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Martin Joseph - 25 Sep 2005 18:57 GMT

> Harmonic vibration from the rear - check the exhaust pipe hangers -
> rubber donuts that support the pipe and muffler. Also check that the
[quoted text clipped - 9 lines]
> breaks I'm going to mount the air cleaner on the body rather than the
> engine.

I don't think you would want to do that, as the air cleaner needs to be
able to move with the engine, or something even more
expensive/necessary will break....

> The air cleaner can is mounted on three rubber mounts. These break,
> usually the rear one sometimes the outer (right side) one. They aren't
> cheap, for what they are, but there's not much choice but to buy some
> and replace them as needed, not
> sooner.

This seems like a good wrecking yard item, but I wonder if a
significant number of these are pre-broken at the bone yard?

Marty

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jlwaller@gmail.com - 26 Sep 2005 05:11 GMT

> The air cleaner can is mounted on three rubber mounts. These break,
> usually the rear one sometimes the outer (right side) one. They aren't
[quoted text clipped - 3 lines]
>This seems like a good wrecking yard item, but I wonder if a
>significant number of these are pre-broken at the bone yard?

you can find some good ones at the unk yard, but eventually they go
bad, if you want new ones, the cheapest i found are
http://parts.mbz.org/ they have very good prices, and i have talked
with the guys that work there on the phone (i live in Florida) and also
emailed them, the guy who runs it, Rusty, has been a MB diesel mechanic
for something like 35 years, extremely helpful, even if you dont buy
anything. I spent like 2 hours on the phone with him troubleshooting
an issue i was having with my Dad's '82 300SD with the Air cleaner.

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