Off throttle under car vibrations

Hi,

I put in a 5 speed in my fjord Si last year and reused an automatic drive shaft. Since that wasn't a perfect match when it comes to length - I recently had a standard DS shortened and brought to the right specs (new U-joints, new splines, front bearing and centre bearing) from a well known company in town with excellent references. Now I have a slight vibration when going off throttle above 60-70 mph. As soon as the speed is below 60 the judder stops. If giving a little gas it's almost gone. I put in a new guibo last week to cure it but that didn't change anything... well maybe a little.

The crossmember and all the other hardware is E12 spec, which should make it fit without custom work. The alignment seems to be OK, I made some small changes to that and it cured some vibrations which ocurred at approx. 35 mph. The centre bearing is pre-loaded forwards as it should.

At the same time I changed the half-shafts but I'm not sure if I got the new ones fitted as they used to be in the donor. Could it be the CV-joints in the shafts?

Any thoughts?

Motor mounts in good shape and tight?

Mike, you might have a point there. I'm sure they are more than 20 years old.

How would weak motor mounts contribute to the vibrations ocurring when going off throttle at higher speeds?

DerSchwede said:

Mike, you might have a point there. I'm sure they are more than 20 years old.

How would weak motor mounts contribute to the vibrations ocurring when going off throttle at higher speeds?

Click to expand...

Under acceleration one mount is in tension and the other is in compression due to the torque of the engine acting on the drivetrain. When you let off the throttle, the situation is reversed.

OK, I see.

I just talked to Mr. Nesch. I'll stop by at W&N on the way home from work tomorrow and pick up a new pair.

Thanks for the hint.
Cheers
Anders

Three things can change the alignment of the drive shaft

Mike Goble said:

Under acceleration one mount is in tension and the other is in compression due to the torque of the engine acting on the drivetrain. When you let off the throttle, the situation is reversed.

Click to expand...

Position of the engine/transmission (angle and displacement), position of the diff (angle and displacement), and fitment of the center bearing. That's why condition of engine mounts, trans mount (and the correct adjustment) are important.

This is not an uncommon problem. Had it on my E39 540iT, and dealer was never able to completely remove it. Never experienced it on an E9 or E3, so at least you know the desired state is achievable.
John

Might be the shaft balance, I had a similar problem in my old race car, drive shaft was shortened etc by "reputable company" but turns out there balancing just wasn't up to scratch, took it else where and had it re balanced and problem gone.

Hope I am not stealing the thread but this is perfect timing since I was going to question motor mounts as a cause for my rough idle ( the engine itself idles smoothly and every other thing in the car seems tight, mounts may be original). Is there any experience with OEM versus poly since both are available?

My take

I too have a 5 spd in my 2800 and there is a shimmy or vibration during some acceleration, not off throttle...I have dealt with it for the 8+ years I have had my coupe but would love to sort it one day. Please keep us posted as to what you find.

Thanks.

nf

'70 2800CS
'72 Tii
'79 911SC
'82 911SC

Last saturday I changed the engine rubber mounts and adjusted the driveshaft accordingly since the old rubber mounts showed quite a degree of deformation. I was just able to make a short test drive before leaving for this week's work trip to Sweden but the vibrations are now less irritating but somehow I have the feel they are still there... I'll look into the halfshafts on Thursday when I'm home again.

Cheers
A

I had a similar E9 problem with vibration starting at 40 mph (not gear dependent) after installing a 5 speed. Vibration was especially bad when cold during the first 5 minutes of driving. Had the mechanic send the twice shortened shaft (measurement responsibility screw-up) back for rebalancing. Replaced the old donut coupling because the vibration was more apparent when cold. Problem solved. Vibration on deceleration seems likely something else.

Did you cut the end of the tranny output shaft or drill the end of the driveshaft when you installed it? If not your Guibo will be be stressed off center.