dr bob

Transmission mounts still holding the gearbox up at normal height? Is the long bolt through the shock and lower control arm correctly tightened? Correct torque on the nuts on the stub axles?

Sagging gearbox mounts allow the trans to move around in reaction to torque load. The long bolt/pin is critical to maintaining the hub relatively square with the half-shaft, and also needed to maintain toe and camber. Correct torque (300+ lbs/ft) is critical for keeping the stub shaft square through the middle of the rear wheel bearing.

mkhargrove

I might be way off base, or maybe I missed something....but what shape are your motor mounts in????

mkhargrove

BTW, as far as boots tearing at the same time....my boots were not so new, and I jacked the car up one side at a time. Before I jacked it up, I didn't notice any tears. Afterward, both boots were torn. I think that if the boots are at that stage in life (rigid/brittle), the change in angle might be enough to cause any tiny cracks to open up. But I'm just guessing....

marine928

New volvo mounts in 2016, suspension refreshed, transmission mounts replaced as well.

Thanks

Jason89s4

OP, mileage? And how many miles have you put on it.
Also see attached page from WSM on engaging "D" (Interesting note about a 2 second delay is "normal")
On my '89 and my '86.5 I also have an inch or two of "roll" after I put it into park. Not sure why, or if normal.
-Jason

marine928

Hi Jason, my 928 has 121k , always serviced no deferred maintenance.
Thanks
Lon Davis

Taguid

Mine has always had a small vibration when taking off from a stop. It quits soon after I start moving but nothing like you describe. I think I might need a new harmonic balancer since I too have changed just about everything else. Hopefully, Dave will have the answer since you've responded with mileage's.

marine928

Update, today I ran the car through different situations to identify under what conditions the shudder happens. Here is what I found, hopefully it helps.

There is a definitive correlation between low rpm's and gear selection. When I start from a dead stop in D and apply normal throttle I get minor shudder as the transmission goes from 2-3-4. If I am in town where speed is limited to 40 and I am accelerating slowly, the shudder is more pronounced until higher speed/rpms are attained.
If I manually shift and hold the shift point at a higher rpm the shudder is very minimal. ( I think that if I select 2nd the trans drops to 1st which produces no shudder at all.)
In manual 3rd gear at 40+ acceleration both normal and spirited produces minimal shudder
In 4th gear, at 35-45 acceleration normally produces pronounced shudder.
In 4th gear at 50-55 normal acceleration produces minimal shudder.
Auto shifting from D at slow speed/rpm produces shudder in all gears with 4th gear producing the most pronounced.
Adversely, doing +60 and applying hard acceleration also produces momentary shudder. Also, doing +60 and applying gradual acceleration produces minimal shudder.
I can feel it through the steering wheel and its feels like it is more in the back half of the car
Any thoughts?

Thanks
Lon

mkhargrove

is it a quick type of vibration, or more like a shudder (sort or like you might get when you drop the clutch from a standstill in a standard without enough revs....not enough to kill it but enough to make the car shake...?

marine928

Hi Kevin,
More like the latter, and it decreases as the torques/speed/rpm equal out. At higher speeds and moderate acceleration no shudder. If I am at 75 mph and apply 50 percent throttle no issues. At 60mph and 50 percent throttle slight shudder until speed/torque equals. At 60 mph and 20 percent throttle, no shudder. Hope that helps.

Thanks,
Lon

mkhargrove

Are you pretty confident that its not an issue related to your torque converter?

Constantine

Hi Lon,

Following along as to your progress to find the source of your vibration/shudder problem.

Interesting about your speed and throttle inputs and how it affects the felt shudder which now sounds like a possible TC engagement problem.

Currently involved with our new project for the C7 Corvette and they have a problem with shudder in the new automatic A8 transmission which has been fixed with a new Mobil 1 fluid, which also requires the transmission to be flushed before adding in the new fluid. The A8 transmission has also been reported with the same problems in the other GM vehicles it is used in, so the shudder problems are not only in Corvettes. There is a bit more to the A8 shudder story, but your problem sounds like a TC/transmission problem more than anything else at this time.

You said the TC was done during the rivet replacement but that is only for the rear flex plate, nothing is done to the TC itself.

Do you know what fluid was placed into the transmission after the TC flex plate rivet replacement was done?

Maybe a trans fluid/filter service at this time would be warranted to try to take out the TC/Trans from the equation? Also a good time to make sure the most up to date fluid for your transmission is put back in? Shouldn't be that expensive or hard to do.

HTH.

marine928

Good morning C!
I'm out for my usual early morning drive. It's the best time to enjoy our cars with little or no traffic. You can really relax. I'm in my 240z and am not driving the 928 until the issue is resolved.
Very interesting about the Corvettes. I will perform all the things you suggested and update later in the week.
Is the vette set up similar to the 928? TT, shaft, and distance between motor/transmission? Definitely a case for study.
Thanks to you and everyone else for the help
Lon

Also I have 131k miles not 121 as I misstated earlier

Mrmerlin

I would go along with Constantine,
replace the trans fluid and filter drain the TC as well ,
you will need 9 qts to refill

marine928

Thanks M,
Hopefully it will be that simple. At least I got some new half shaft's out if it.....