I was driving my 71 cutlass on the highway yesterday at about 75 mph, and when i took my foot off the gas, the steering wheel shook violently as the car decelerated. I tried to replicate it but it would not do it again. The front end is low mileage but about 10 years old. The shocks, transmission, rear end, and driveshaft are new and the 455 is a recent rebuild. Anyone have any idea what might have caused this?

 

Vibration or violent side to side oscillation?
The latter is very common in the 4x4 world. It is caused either by worn steering components, or bad steering geometry, mainly insufficient caster. Or often a combination of both.

Look for "death wobble" videos.

 

Any recent alignment? To echo the above these cars w radial tires can benifit from increased caster setting VS the original caster settings

 

I am just going out here and make a guess that it is the idler arm.

 

The shaking was not side to side and the car did not lose steering.control. more of a vibration like out of balance tires, but more extreme, so hard to call it a vibration. Alignment was done with the replacement of the front end parts. The car sat for 8 years shortly after the new front end was installed, so I needed to replace the tires due to flat spots from sitting. The tires were very low miles that needed replacement. I put same Cooper Cobras on the car. The car drive straight otherwise. I'm not familiar with caster settings. I was also wondering if the problem could be in the drivetrain since it started simultaneously to taking my foot off the gas at 75 mph.

 

I would begin by removing the front wheels/tires with the car on jack stands. During the removal pay attention to the lug nuts. Are the lug nuts tight (before loosening for removal)? Is/Was there any free play? Thoroughly examine each tire & wheel. Examine the tires for bulges & tread separation. If you can convince yourself the tires/wheels are OK, move onto the steering assembly/linkages, idler arm (already mentioned), rag joint, etc.

To answer your question regarding drivetrain producing vibration the answer is yes. An unbalanced drivetrain can cause vibration. There are some caveats, however. Generally, the vibration will remain consistent within a speed range - generally. The vibration most often can be replicated again and again while driving - generally. Most often, however a vibration w/in the drivetrain is isolated to the frame/chassis - generally. It can be serious enough to transmit into the steering column; yet, drivetrain vibrations generally remain isolated from the steering - you can generally feel the seat vibrating not the steering wheel - generally.

I’ve owned three vehicles which developed vibration as the result of one (generally) or two failed U-Joints. The vibration in all cases was significantly more pronounced during deceleration. If the movement is definitely in the steering column (wheel), I would suspect the vibration movement to be a up & down vibration if it was related to any portion of the drivetrain.

 

Since it occurred as soon as you let off the throttle, I’d say add the u-joints to your list of things to check.

 

I vote for the U-Joints, especially since it happened when you let off the gas. Did you perhaps get hard down shift at the same time, especially if you have an overdrive trans, I've had some that had wonky shifting for no good reason on early GM OD trans.

 

checked the front and and alignment. Both good. A little play in front wheel bearings so those will need to be replaced. Doubt that would make the steering wheel shake. And it was an up and down movement. Now that I'm keying in on it I think I can notice the vibration at speed coming off the throttle. Will need to check u-joints. Maybe wheel balance.

 

Take a look at the rear pinion nut too! After i got mine home i found like 3 turns loose! Didnt apear to have been worked on. I was there replacing the u joints.

 

I have never seen bad ujoints make a steering wheel shake. The problem is in the front end.

 

Google "death shake" or "death wobble" there are lots of posts about this.

 

If you can't replicate it, we have to rule in pavement anomaly.

 

I had a 78 dodge power wagon back in the day. I was driving it down the highway and took an exit. On decel down the off ramp it started a harsh shuddering vibration halfway down the ramp there was a loud bang. the front driveshaft ejected out the driverside across the ramp all due to a bad u joint. Drive the car slowly in reverse with the door open and see if you hear a squeaking sound. Thats a sure sign you have a dried out u joint.

 

I looked up symptoms of a bad u joint. When I put the car in reverse with my foot on the break it makes a clunking sound as indicated as a symptom. I replaced the driveshaft with the swap to the th400 so the u joints are fairly new. I will need to check.