Greetings all, I have a problem with my Shiatzu vibrating Vette. The subject is a 1996 LT4 6 speed with 83,000 miles. It had what can best be described as a series of vibrations at higher speeds, some seemed to come and go some were persistent high frequency and low, so here's what I have done so far:

I bought brand new wheels and tires and road force balanced them... twice. I then checked the front suspension and steering and found the front tie rod ends to be bad and so I replaced them. The front bearings tested OK as did the upper and lower ball joints, the bushings are old but still adequate. Shocks need replacing but could not cause the vibration so there were not replaced yet. Took off the front calipers made sure the pistons were moving properly, And that there was no issues with the rubber lines. Later tests were performed after a long drive and temperature readings from the bearings didn't fluctuate more than five degrees the same with the calipers and the rotors. This test proves there is not a sticking caliper or front bearing issue. Onto the rear, right rear wheel bearing was very bad, I replaced it as well as the half shaft U joints as my fear they would have taken some additional force there. The left rear bearing checked out OK as did the half shaft U joints. I found that both tie rod ends in the rear were very worn and replaced them both. The rear bushings are old but adequate, the rear brake calibers were removed and tested for proper piston movement and slide movement with no issues found with either, nor were any issues with the rear rubber brake lines. Later testing would show temperature readings on the rear bearings, calipers and rotors to be within a few degrees of each other and nothing overheating. There was acceptable play from the prop shaft U joints and rear end, no issues expected with either. One thing I did notice was there was a lot of free play with the differential, that is to say, grabbing either rear wheel when in the air and turning it, there is a bit ove free movement as you spin the wheel before it engages with the differential. Im not sure the technical term for that, nor how much it should have. So this is still up in the air so to speak. Upon a test drive I found that most issues were either resolved or mitigated except for one vibration that comes in above about 75 and gets worse up until 83 then smooth out a little above 83 but never goes away. I described this vibration as a low frequency vibration felt in the entire car, the seat the steering wheel the dashboard the rearview mirror the whole car shakes as if a tire were out of balance. OK so for fun let's take the old stock original equipment wheels, and the Goodyear tires that were on it, which we're still in good shape, and have them road force balanced and put them on and see where we are. And it should be noted that the tire pressure sending units were put on to the new set of wheels and tires, so the original equipment wheels and Goodyear tires that were on it do not have the sensors in there. After a test drive, I found exactly the same vibration, so it does not matter if it's the brand new wheels and brand new nittos or the original equipment, the vibration is still there. OK so at that vibration apex which is 83 miles an hour, I push in the clutch and the engine RPM's dropped to idle and the vibration is still there no change. At that speed 83 miles an hour, I can apply torque in either 5th or 6th gear it doesn't matter, and the vibration remains the same and does not get any more intense regardless of whether I'm applying torque, only, whether I'm applying break along with torque, or whether I'm in neutral and applying brake only it's all the same.



Its Saturday and I want to continue working on this problem. However I have no idea what to do next. I've watched scores of YouTube videos, read scores of forum blogs etc, and I have done everything they have suggested but clearly have not found the solution.

Your thoughts? Questions? Thanks for reading this far!! Dude

 

I've got a '94 M6, and have been fighting various vibrations as well, but there was one in particular that I was able to resolve, unfortunately it doesn't quite sound like yours. I had a high frequency vibration that started around 50, and got worse as speed increased, however, it was at it's worst under float conditions. For example, steady 75mph, slightly there. Full decel, slightly there. Just enough throttle to unload the driveline (slight decel), or throw in the clutch, bad high frequency vibration felt through the shifter mostly. Replaced all six u-joints, no change. I noticed what seemed like excessive radial play at the front yoke. Figured the tailshaft bushing was worn. Called ZF Doc, and he said that it's worn splines in the yoke, not really the bushing. Didn't really believe him, but changing the yoke is easier than taking the tailshaft housing off the trans, so I ordered the *special* yoke that he offers for the ZF. Well I'll be damned, zero play with the new yoke, and the odd vibration was gone. Looking at the splines of the old yoke reveals no visible wear, but the play is gone none the less. I figured that was the bushings job, but I guess not. Hope this helps.
Scott

 

Scott, thank uou so much for the information. So it was the worn splines in the yoke!?! Okay Im not sure what the yoke is but give me an hour and ill check it out! Thanks, I have somethung to do now!

 

Something else worth checking-I would put new u-joints on the driveshaft and then have it checked for balance.

 

I agree with Scott. Check the play in the slip yoke where the driveshaft inserts into the transmission. You'll need to drop the exhaust. Also the driveshaft U joints that were previously mentioned.

 

Drop the exhaust and drive shaft... that sounds difficult and complicated and ultimately expensive!?!. Yep, that has to be it then, id say you guys nailed the problem. The ZF Doc site is outstanding, and checking that yoke play or whatever and U joint (other than get into the diff) is the only thing I havemt done yet. Okay that will be the next step! Much appreciate the advise!!! Dude

 

In your first post you say "Shocks need replacing but could not cause the vibration so there were not replaced yet." I would not rule them out. Back in the day, I bought a brand new Oldsmobile Delta 98 Regency. After about five months, the same type of vibration you describe appeared. The dealership checked everything and after about seven or eight trips to the service department, I asked them if they had checked the shocks. I was 21 and the service writer looked at me and rudely said "How can it be the shocks - the car is five months old". Well, guess what! As a last resort, they pulled the shocks and the front ones you could run them by hand like a slinky. The front tires had been "dribbling" at certain speeds and conditions. The feeling of serving that ***** some crow was immensely satisfying. Not saying this is your case, just something to consider.

 

What year is the car in that video?

 

It's a 93 6 speed with 105k miles. I believe all the 6 speed slip yokes are the same. This slip yoke was causing vibration like you described which was resolved when I replaced it. I own 7 MN6 C4s and have found that at around 100k miles it's best to replace all 6 U joints (4 half shaft & 2 driveshaft) and check the slip yoke for lateral play. You can check the slip yoke with the C beam in place. You'll need to drop the exhaust from the manifold back to check it. And since you have vibration it makes sense to remove the C beam and driveshaft while you're there and do the U joints.

Also, don't be intimidated. Dropping the exhaust, C beam and driveshaft is pretty straight forward. Once the cars in the air you should be able to do it in an hour or 2 provided no bolts are seized up.

 

And if you slightly bend a pinch welded piece of metal in the area, you can remove the driveshaft without touching the C beam. It'll be obvious where you need to bend it. The exhaust should come out though.

 

Right. Exactly what MaxLean just said. Bend the pinch weld and keep the C Beam in place. That's the quick way to do it. Easy job.

 

Okay, just so that I am sure, to perform the inspection like to one in the video, I will need to drop the exhaust only, and if there is the play as seen in the video then replace the slip yoke and both U jioints, (and I know I should anyway but...) if there is no play or acceptabel play (unlike that in the video) then that is NOT the problem correct? Thanks!!! I will end up replacing everything under the car eventually, but i want to find the specific cause of the vibe first, before firing the parts cannon at it. Thanks for the theoretical approach and proper testing procedures guys!

 

Thank you for the insight! Yes I will be replacing the shocks as well as everything else under the car eventually. And I also agree that whatever is causeing the vibration is being amplified due to worn out shocks. But from my understanding ( and im just an enthusiast no where near a mechanic) the only thing that could cause this low frequency vibration as described is something that is rotating. And as its speed dependant and not RPM dependant and present regardless of the torque applied or brake applied, I would think that would rule out engine or brakes as a cause. As for a dribbling tire, you should be able to remove the shock absorbur completely and NOT have a dribbling tire if the tire is balanced.

 

Not so. That is the function of a shock absorber - to limit and control vertical movement. You can have your wheels balanced all you want but with no shock, what is controlling the vertical movement of the suspension/wheel? If you hit an obstruction in the road, the wheel/tire is going to be forced up against spring pressure. The spring forces it back down and somewhat compresses the tire - balance or no balance. Race cars have adjustable compression and rebound.

 

I had a 93 Vert that had a slight vibration when driving. I had my tires road force balanced, checked for play in my wheel bearings. No movement . I tried another set of known good wheels. Still had the vibration with my car. It only had 56,000 miles. I replaced the front wheel bearing and the vibration was gone.

 

Yes I agree!! I imagined a smooth road!! Im so nieve.

 

Correct. But I recommend replacing the U joints regardless of the condition of the slip yoke. There's no good way to test the U joints short of just replacing them.

 

Thanks for the help! Okay as I understand you checked the bearings for play at first and didn’t notice anything concerning, then you swapped a set of known good wheels/tires (you had previously road force balanced the original wheels/tires) and still the same vibe thereby eliminating the wheels/tires. My question is, did you re-check the front bearings or just replaced them. I know either way it fixed the problem, the reason I ask is that I myself, the Dude, or you can call me Duder, or His Dudeness, or Elduderino if not into the whole brevity thing, did the original “check out” on the front bearings and didn’t find a problem. Im starting to suspect that I really don’t know what I’m doing. Thanks for any more insight!!!

 

Yes, agreed, while im there and will do the pinch weld trick too, thanks for that one MaxLean!

 

Follow up. Without removing the exhaust I did my best to check play and movement with a pry bar, and it all seemed to have perfect tolerances. I honestly was dissapointed that I had not seen something obvious. But it got me thinking...I reread MaxLean's post carefully and he described a high frequency vibration load dependent intensity. That explanation makes perfect sense for the slip yoke or a prop shaft U joint as it spins 3.5 times faster then the wheels and would present a higher frequency vibration. My problem is a low frequency vibration that is not dependent or affected by torque load breaking or engine RPM. And MaxLean even said in the beginning of his explaination of his vibration that "unfortunately it doesn't quite sound like yours". OK well darn, now I'm back to where I was in the beginning, I think.