Ben Taylor

My 81 automatic shakes when I get up to 60 or higher then let off the gas. If I'm giving it gas the vibration goes away. I recently replaced my front brake pads and rotors, could that cause this problem? What else could be causing this?

...Roger...

I would be looking at the driveshaft and on back. Check the out shaft bushing in the back of trans. Can you reproduce any of the vibration sitting still in drive by reving up and letting it come down ?

7T1vette

You may have a U-joint going bad (or worn out). Check for 'play' in the joints on driveshaft and half-shafts. It could also be a rear wheel bearing or wear in other rear suspension components.

Ben Taylor

There is no vibration when I give it gas in gear with the brakes on while stopped. All of the u-joints look good, I couldn't see any play in them. I also noticed that if I'm above 75 there is no vibration, so it only happens from 60-75mph. So it seems to me like this is a balance issue. What would be out of balance only while decelerating though? Could it be one of the rotors?

...Roger...

Sorry I meant to say can you reproduce any of the vibration sitting still in park or neutral and revving up and letting it come down ? Not high revs. One of the first things I look for is water pump and fan vibrations ,sometimes the fan can transmit some weird vibrations.

63Nova

Sounds like a bad u-joint.

jfleye

Out of balance tires can do some harmonic vibes.

mrvette

Where ever this takes you to, you gotta find it FAST too....

that could be serious....gotta cover your *** on it....

Goody

If you think about what you posted a bit ago you know it is balance related so it is in the drive train....The speed it happens at is dependent upon driveline RPM and not engine RPM. The driveshaft and halfshafts are spinning at the final drive RPM and not the engine RPM so the vibration being there from 60-75MPH indicates it isn't the engine else the vibes would be there regardless of gear selected.

A few of us are having this issue, myself included at the same speeds as you are, my C3 is a 4spd '81 with 72K on it now.

For good measure I have replaced the shocks and rear spring which has eliminated more than half of the vibes. My original GM shocks were useless at damping anything. My thoughts were the rear was sagging too much allowing the halfshafts to bind a bit. Maybe the damage has already been done and a ujoint is now bad. The diff has a leak around the right yoke seal so when I rebuild the diff I will address the Ujoints thereby eliminating another potential source of the vibes.

Just a thought on my recent experiences.

no2pencel

I have the same problem on my 77. I have replaced all of the u-joints with NO success. The 3rd member mounts to a bushing and may be worn and the problem (Binding). It could also be the motor mounts.

...Roger...

Don't forget driveshaft angle alignment. Do a search.

beige79

I know this thread is more than a year old, but I was wondering what ended up resolving this vibration issue? I am having the same problem now and I was thinking u-joints, but would hate to change them all if that is not the cause of the vibration. Thanks.

~ Gary

sly vette

I'm curious to the answer also.
Interesting that the vibration only occurs on deceleration.
Poss. trans mount or pinion snubber bad allowing the angles of the driveline to change???

babbah

beige79

In my case, the tires were replaced and balanced two weeks ago and the vibration problem existed prior to the tire change.

A couple of other things about my particular car -

• The speed at which this occurs does not really matter. It does it all of the time, even at low speeds.

• Although my car is 31 years old, it has only been driven 18,500 miles. I suspect that whatever is wrong with it may be more related to deterioration from just sitting and not parts worn out from high mileage. Of course, the part(s) may just be defective with no relation to age or miles.

RIO68

I did a search because I have issues with mine. Man, what a can of worms that search turned up. More ideas and opinions than Carter has pills to sell. Vibrations on these cars have apparently gone out the roof and I guess most of it is because over the years we have come up with all kinds of modifications to drive lines--FI motors and trannys and gear ratios etc.--which have given us more options and thus moved past where the original engineers ever thought about.

All I did was go to a 700R4 and little things began to pop up! I think mine are going to be drive shaft angles (maybe BTO crossmember) when all the smoke clears?????
Every bolt on mine has been turned except body mount bolts.

dembo

Just wondering, When you shortened the driveshaft, did you have it rebalanced as well?
Nick

BPHORSEGUY

Friend just had exact same symptoms on a Tahoe it was the drive shaft rebalance that fixed it.

RIO68

not mine--it was re-balanced

slowride262

I've seen were u-joints looked good but when taken apart they fell to pieces and were bad.
Also seen drive shafts that went out of balance from just rust inside of them.
But a bad pinion bearing will also make a vibration at them speeds.