Mistery vibration and other illnesses

[Geronimo '74]

First of all, I did use the search button and I found out a lot about vibration issues. But my mystery is not solved.

Here's what's wrong with my car as far as I can tell. It takes in false air between intake and cylinders. So I need to replace those gaskets . My gearbox (915) needs a rebuild. and my front tires are unevenly worn due to a bit to much neg camber.

Here are the symptoms. When I drive, no matter the speed, I hear a low tone growl front and left. Like I'm running an off-road tire there. A higher speeds ( 80, 90, miles an hour) the growling noise gets louder and I also feel an oscillating vibration, off and on for a few seconds. It is noticeable on the passenger side, next to the wheel-well. But also in the gearstick, in the steering wheel..

At first thought, I would say the wheel bearing. Did the test of swaying from left to right, to see if there's any difference in noise. No difference in noise, so I ruled out the bearing issue.

What is more is that I noticed that the speedeometer needle is bouncing a little bit in the same oscillating rhythm as the vibration. It goes down about 5 to 7 km/h and then goes up to the correct speed again, all in the same rhythme as the noise. (Well maybe not exactly synchronous but it is obvious they are linked somehow.)

So now I'm thinking gearbox... because of the bouncing speedometer needle. Can gearbox issues cause this type of vibration as well?

A friend mechanic drove it and he did not think it was a bearing issue but frowned when I asked if it could be a gearbox issue.

Any comments, thoughts, directions would be very welcome.

Thanks you guys.

[Oh Haha]

Obviously hard to diagnose at the keyboard but them me throw a couple of things at you:

Are the tires directional and on the correct way? Yes, it happens.

How old are the tires? You can get strange sounds from worn tires.

Are the rear tires worn in a funky pattern?

Do you have any grinding going up or down through the gears?

Does the speedometer fluctuate with the tach?


I'm heading out for a drive in mine for a few hours. I'll check back when I return. Not sure of the time difference in Belgium but maybe we can come up with some new info today still.

[stlrj]

Rear wheel bearing?

[Steve W]

Bad tires can sound like bad wheel bearings. Try swapping the wheels around either side to side or front to back temporarily to see if the noise goes away or moves to the other side.

[Geronimo '74]

Originally Posted by Oh Haha

Obviously hard to diagnose at the keyboard but them me throw a couple of things at you:

Are the tires directional and on the correct way? Yes, it happens.

How old are the tires? You can get strange sounds from worn tires.

Are the rear tires worn in a funky pattern?

Do you have any grinding going up or down through the gears?

Does the speedometer fluctuate with the tach?


I'm heading out for a drive in mine for a few hours. I'll check back when I return. Not sure of the time difference in Belgium but maybe we can come up with some new info today still.

The tires are directional and mounted correctly.
They're Bridgestone SO2, about 2 years old. Front worn more on the insides due to negative camber.
Rear tires worn evenly, everything looks normal at the rear.
Gearbox flaws: sometimes dificult to engage 1st and 2nd, teeth grinding when going to 3rd too fast. whining when at low speed in higher gears.

I made a mistake in my first post here. It is not the tach needle that bounces it is the speedometer! The tach is steady (as it should be at a constant speed). It is just the speedometer that is acting up.

And thanks already for the swift reply, that's what makes this board so great.

[Geronimo '74]

Originally Posted by stlrj

Rear wheel bearing?

Play on rear wheels is minimal.

[Geronimo '74]

Originally Posted by Steve W

Bad tires can sound like bad wheel bearings. Try swapping the wheels around either side to side or front to back temporarily to see if the noise goes away or moves to the other side.

The tires are directional, can I swap them from left to right? (even only for testing puposes?)

Wheels are 7 and 8s, I think they won't fit on the front, if I switch front to back.

[ischmitz]

First off your message is getting "lost in translation" a little bit: The German word "Tachometer" refers to the speedometer (showing car speed). It is driven off the transmission via an impulse signal. If the readout drops while the wheel speed stays constant you might have an issue with system voltage changing.

The American word tachometer refers to the rev. counter (engine speed). You seem to indicate that your speedometer drops by some value 5 - 7 km) when the noise occours, correct?

Thinking out of the box could the periodic noise be your AC-compressor kicking in with a bad bearing or similar. Maybe that in combination with some poor GND causes your board voltage to change. The compressor uses a magnetic clutch that can either be bad (drawing too much current) or simply bog down the board voltage due to issues elsewhere......

But then agian it is really difficult to diagnose something remotely. A good mechanic familir with 911 should be able to narrow things down pretty quick especially when you can provide solid observations of what happens.

Good luck,
Ingo

[Geronimo '74]

Originally Posted by ischmitz

The American word tachometer refers to the rev. counter (engine speed). You seem to indicate that your speedometer drops by some value 5 - 7 km) when the noise occours, correct?

-->CORRECT!


Thinking out of the box could the periodic noise be your AC-compressor kicking in with a bad bearing or similar. Maybe that in combination with some poor GND causes your board voltage to change. The compressor uses a magnetic clutch that can either be bad (drawing too much current) or simply bog down the board voltage due to issues elsewhere......

--->I don't have AC in my car.

But then agian it is really difficult to diagnose something remotely. A good mechanic familir with 911 should be able to narrow things down pretty quick especially when you can provide solid observations of what happens.

Good luck,
Ingo

Thanks Ingo, The mechanic that drove it, said he hasn't seen this before and could not give me a diagnosis on the spot...

[911pcars]

To help isolate the source of the noise, run the vehicle up to speed, then shift to neutral. Noise still there?

Sherwood

[Geronimo '74]

Originally Posted by 911pcars

To help isolate the source of the noise, run the vehicle up to speed, then shift to neutral. Noise still there?

Sherwood

YES! whether the car is in gear or not, makes no difference, noise remains!

[Oh Haha]

Anyway you could swap wheels with another 911 owner to see if it goes away?

Your front tires being worn with negative camber could be the cause.


EDIT--Belgium isn't THAT big, is it?

[Steve W]

Originally Posted by Geronimo '74

The tires are directional, can I swap them from left to right? (even only for testing puposes?)

Wheels are 7 and 8s, I think they won't fit on the front, if I switch front to back.

Directional or not, you should be fine just for testing, so left to right is no big deal. Guys have fit 8s and 9s on the car, so I think you're also ok to swap them front to back. It's not like you're going to leave them on permanently.