Desperate for help...vibration

[997at]

Hi All. Really hoping I can get some advice on where to start with this issue from the professionals and super experienced here. I have an 82 SC US spec. I'm getting a vibration that I can feel through the whole car in 4th and 5th gear only when under throttle. Engine idles smoothly and revs smoothly to red line in neutral. It's driving me crazy!

[Oneredspyder]

So I would ask for more details. Under throttle at all rpm ranges? Does the vibration stop if you take it out of gear? Is it at all speeds? Is this new and have you done any work on the car recently.
Any “funny” noises?

[997at]

Originally Posted by Oneredspyder

So I would ask for more details. Under throttle at all rpm ranges? Does the vibration stop if you take it out of gear? Is it at all speeds? Is this new and have you done any work on the car recently.
Any “funny” noises?

It's under throttle at all rpm ranges but only in 4th and 5th gear. It does seem worse with more speed and more throttle. Vibration goes away completely if I lift off the throttle or put it in neutral. Present at all speeds that are relevant for 4th and 5th gear, so from about 50mph+. The gearbox was just rebuilt by a very experienced mechanic but the vibration was there before I had the gearbox rebuilt. It seemed to show up randomly about 4 months ago. Car has 85K miles on it. No funny noises.

[stownsen914]

First things I'd want to know is whether it is speed dependent or RPM dependent. What happens if you are 50+ in 4th gear while feeling the vibration and you downshift to 3rd gear while maintaining 50+ mph? Vibration stays or goes away?

If the vibration was there before the rebuild, the rebuild didn't cause it, but it doesn't mean the gearbox isn't the source.

Where do you feel the vibration most? Steering wheel? Through the seat? If you have a passenger in the car, do they feel it too?

[997at]

Originally Posted by stownsen914

First things I'd want to know is whether it is speed dependent or RPM dependent. What happens if you are 50+ in 4th gear while feeling the vibration and you downshift to 3rd gear while maintaining 50+ mph? Vibration stays or goes away?

If the vibration was there before the rebuild, the rebuild didn't cause it, but it doesn't mean the gearbox isn't the source.

Where do you feel the vibration most? Steering wheel? Through the seat? If you have a passenger in the car, do they feel it too?

If I'm in 4th at 50+ and downshift to 3rd the vibration goes away. I'll do more testing today to see how speed dependent it is in 4th gear. I think I feel it more in the seat and as if it is coming from the engine/gearbox but I can also feel it through the steering wheel. Haven't had a passenger in yet but will try to get one.

[sugarwood]

No vibration when you let off the throttle? Coasting at 60+ has no vibration, right? Would that rule out a tire balance? How old are the tires? Visual inspection for a bubble?

[jlex]

Separation in the tread of the tire? Check for roundness...

[speedster911]

did you replace the engine and trans mounts during your rebuild?

[porsche930dude]

Was the clutch inspected when the gearbox was out? you definately need to rule out the tires and wheels before digging any deeper. Also check the cv axles for tightness

[Oneredspyder]

I would not drive it fast at this point. Start with the simplest. Look and feel the tires. Jack each corner up and rotate and shake the wheels horizontally and vertically. Sounds like you think it’s in the rear. Put the car on 2 jack stands under the torsion bar/ swing arm bushing and try to move every rotating part, axles, etc. look for looseness.

[SoyRacer]

If you haven’t already - replace the transmission mounts
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Very commonly the cause of loaded vibrations.
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It would also help if you respond to the people on this thread that are looking to help you diagnose this.
-
RR

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[dannobee]

In the industry we use an Electronic Vibration Analyzer (EVA) when we run into unusual/uncommon vibrations.
Before anything, we "do the math" on tire rollout (circumference), gear ratios, etc. There are various worksheets to help you. Then road test the car with the EVA's magnetic sensor attached to something steel and solid, like where the seats mount. The EVA will capture the frequency and amplitude of the vibrations. Compare to your worksheet, then zero in on what component can cause the vibration at your certain speed.

Before EVA's we used "reed tachometers" to pinpoint the frequency of the vibration. But those are likely far more money than an EVA off of ebay or craigslist.

[997at]

Originally Posted by sugarwood

No vibration when you let off the throttle? Coasting at 60+ has no vibration, right? Would that rule out a tire balance? How old are the tires? Visual inspection for a bubble?

Yup. Coasting at 60+ has no vibration at all. Tires are only 2 years old.

[997at]

Originally Posted by jlex

Separation in the tread of the tire? Check for roundness...

I was wondering about that too but if I lift off the throttle or coast at speed there's no vibration at all.

[997at]

Originally Posted by speedster911

did you replace the engine and trans mounts during your rebuild?

Yup. Tranny mounts are brand new. I replaced the engine mounts with OEM about 4 years and 5K miles ago but I'm wondering if one could have cracked or broken?

[997at]

Originally Posted by porsche930dude

Was the clutch inspected when the gearbox was out? you definately need to rule out the tires and wheels before digging any deeper. Also check the cv axles for tightness

Clutch was replaced when the box was rebuilt with a brand new Sachs unit. CVs were disconnected and reconnected as part of the tranny work and were tight before and after the gearbox rebuild.

[997at]

Originally Posted by Oneredspyder

I would not drive it fast at this point. Start with the simplest. Look and feel the tires. Jack each corner up and rotate and shake the wheels horizontally and vertically. Sounds like you think it’s in the rear. Put the car on 2 jack stands under the torsion bar/ swing arm bushing and try to move every rotating part, axles, etc. look for looseness.

I think I'll try to do this in the coming week. Thanks.

[997at]

Originally Posted by SoyRacer

If you haven’t already - replace the transmission mounts
-
Very commonly the cause of loaded vibrations.
-
It would also help if you respond to the people on this thread that are looking to help you diagnose this.
-
RR

-

Yup. Tranny mounts were replaced with the gearbox rebuild. Sorry for the late replies. Took the 991 out for a long drive today. Would preferentially have taken the SC if it didn't have this issue

[sugarwood]

So, are the motor mounts the obvious suspect?

[3rd_gear_Ted]

Check your CV joints is my thought.
Somewhere in the drive train a harmonic frequency is occurring @ certain RPM/load conditions.