Hi all,

I'm getting a vibration from the mid - rear of the car under load - back off the throttle and the vibration goes. I've checked fluid levels, had the wheels balanced and 4 wheel alignment checked to no avail.

BTW, the car is a 79 auto and the vibration starts at about 70mph on a light to medium throttle.

Any suggestions?

Regards

Jon Jeffryes

 

Jon
I think its your car trembling at the thought of having its engine transplanted to the back, that yellow super b##tle you've just acquired has been winding your 928 up!

Yes get rid of the yellow one !

LoL

 

Well, this is a common thing that not many peoploe seem to have a handle on. I'd suggest (just my minor opinion and not based on anything like "expertness") Wheel bearings all around, dinged wheels (mine were and it made a difference), CV joints, tranny mounts, and the bad ones: torque tube and/or torqe converter bearings.

These are just suggestions I've heard in the past. Good luck. - Ruf

 

Does the vibration go away when you go above 70 mph, say to 80 mph? If it does it suggests that it's an harmonic balance problem in a wheel. There are ballancers that will dynamically check wheel balance AND compensate for tire irregularities.
Could also be engine/transmission mounts.

 

Marton!!!

Is there no escape? If you want a real laugh, I've managed to kill the 911 as well, last night the ignition rotor fell apart....fortunately on my drive. I think the PO was a little enthusiastic when he fitted it.

Now be serious, 928 - my first thought is a drive shaft or wheel bearing...any sensible ideas?

atvb

Jon

 

Hi Ernest,

No, the vibration is only there from 65-70'ish upwards under load, back off the throttle and the vibrition goes. On the flat on a steady throttle there is no vibration.

Just driven the car again, if you accelerate HARD, then the vibration cannot be felt!

Wheels have been dynamically balanced last week, plus fronts to back swapped to make sure it isn't a bad wheel.

Regards

Jon

 

Mr Dunks,

that was supposed to be Martin, not Marton...someone moved the keys about on this keyboard :-)

 

Sometimes vibration is a good thing . . . . . sorry couldn't resist

 

Spicing up the message board.....

 

First suspicion would be CV joints. Perhaps it is time to check and repack all four joints...

Another slight possibility would be an engine misfire that feels like a drivetrain vibration.

 

Thanks Wally,

I plan to pull the drive shafts this weekend and have a good look. If that doesn't cure then I'll look at the engine again....although the vibration is speed related - not rev' related - i.e. 2nd gear with higher rev's and the vibration comes in underload at the same speed level.

If that still does not cure the problem...then i'll lock it in the garage and drive the 911.

Regards

Jon

 

Jon,
If you remove the joint shafts, then replace then left/right (I.e. you get new wearing areas)
Vibrations in old 928 is a difficult matter, I am sensitive regarding that and have been fighting with vibration for more than one year - Now my car is silent like a new, only I have some 3000 RPM vibration left.
Here you have list of what I did:
- Tires
- Wheel alignment and roundness of the wheels - The 'telephone wheels' are not as good as the '7 slot flat wheels'
- Engine mounts - And for yours the 2 small shocks
- Front spoiler - An old floppy front spoiler can make a lot of high speed vibrations!
- Suspension (A arms - Bearing joints- Tie rods)
- Alignment - Remember don't lift the car - Better do it your selves (See the article in 928 Forum # 4 or Earls homepage)
- Trans shaft - Check front and rear clamps and release eventual forwards pressure
Good luck with 'your hunting'

 

Erik,

Thanks for the Advice.

I've had 4 wheel alignment (3 times following recent work)

Tyre balance (dynamic, following new tyres all round)

New front drop links

New tie rods (another alignment)

Replaced TT and Gearbox (yep, another alignment) vibration still there

Wheels swapped front to rear and balanced again - no change

New complete SS exhaust with new mountings - no change

New SS tank cradle - no change

So it's definately at the back, not the TT, not the Trans, not the wheels, not the front suspension

Could be driveshafts or engine/mounts related. I'll try repacking and swapping sides this weekend.

Thankis for your help.

Regards

Jon

 

Jon,
Did you check your:
- Joint links in A-arms and the lower joint links
- Longitudinal play in upper A-arms - I solved this by placing a 4 mm thick poly washer 26x60Ø at the rear
- The front toe - If the car was lifted during alignment you have toe out and not toe in (See my article in 928 Forum # 4)
- The rubber mounts for the gearbox - If this is old and 'tired' the gearbox is placed to low - Can be solved temporary with distance washers for lifting the gearbox free of the rear cross-beam
- The rear toe and the condition of the Weissach links
But most properly that is the gearbox mounts
- The engine mounts + for yours (pre 1984 models) the small hydraulic dampers
Good luck with your project - You are welcome to come back if needed - Anyway I am interested to hear the positive result - You will find it one day!

 

Jon,
Any news?
Regards from Erik in Denmark