Shake at speed

[Starman]

Got a bit of a shake at speed on a 91 with 34K miles, just appeared over the last few years. Interested in what others have had issues with. From about 70 mph on, there is a pretty good shake from the rear of the car. Rear spring was replaced last year and the car is adjusted to level. New Nitto tires in May and aligned at that time to spec on a Hunter Laser System - got the print out.

Passes a bounce test fine and no oil on shocks. Seems to be a lot of side to side movement with rear body. I'm curious about experiences with sway bars/bushings, u-joints and other rear suspension items. Getting ready to go through the car and don't want to miss anything.

[Kevin]

do you still have the tire pressure sensors?

[Aurora40]

It's not something simple like a wheel weight fell off, is it? I've noticed those "hidden" weights do not stay on as well as the lip clip ones.

[tccrab]

do you still have the tire pressure sensors?

If one of these has let go then you'll get vibrations at speed.
It would show up on a spin balance, so if you've already had the tires/wheels balanced then this is probably not causing your vibration.
Slop in the rear suspension will add to a vibration, but for the most part won't cause one of their own.
A dry U-Joint can cause vibration at speed. Of course there's 6 of the little buggers, so it's pretty tough to diagnose just one of them.
Rear-end/transmission/driveline alignment is critical. Have you noticed the shifter moving vertically during heavy acceleration? If so, the C-Beam bolts have worked their way loose you could have a resonance that's building from an out of alignment driveline. Re-torqueing the C-Beam bolts might help.

Good luck!!

TomC
"Crabs"

[Jeffvette]

It's not something simple like a wheel weight fell off, is it? I've noticed those "hidden" weights do not stay on as well as the lip clip ones.


Only if not installed properly (cleaning the surface). The only time I have seen one come off has been from excessive heat from track events as it just melts the adhesive. Which is why you use reflective HVAC tape over them.

Onto the vibration. Is it definitely speed related or engine RPM related?

Check:
U-joints
Bearings
Suspension bolts
Worn bushings

[Starman]

Guys, thanks for the response so far. It does have the tire pressure sensors and I only use lip weights on the wheels. The issue is definitely speed, not rpm related. Up to 60-65 mph is vibration free - even in 2nd gear.

I should also mention that the drivers rear tire was chopped when it was replaced, repeating 2-3" long scallops on the inner tread block, camber was off, but wear should have been uniform around the tire.

The problem is also more apparent on rougher roads. This makes me think shock, but I have a good bounce test. Hate to guess at $220/shock. I've also never had a shock failure with the Delco-Bilsteins.

This is C4 number 6 for me and I have run them up to 80-90K miles. Most of the likely culprits like u-joints, wheel bearings, suspension bushings or shocks, I would not expect to see at 34K miles, but the car is 17 years old. I'm particularly interested in unexpected age related failure experience. I have to do some clutch work, so most of this is coming off anyway, I just don't want to miss anything to check while it is apart.