I'd like to say that I've covered the obvious items here, but rather than make an assumption of something simple I've overlooked, we'll assume I've done nothing more than listed.

I'm trying to troubleshoot a driveline vibration that I have had little luck in making a dent in. I'll try to describe what I think I've collected about it, and what I've done thus far:

What I'm seeing:
Worse on heavy acceleration
Seems to increase as RPMs increase over 3500 rpm
Seem to feel it most predominantly in the "seat of the pants"
Specific gear doesn't seem to matter

What I've done:
Had all four tires re-balanced
replaced all U-joints
cleaned and painted drive and half-shafts
replaced T-arms
had rear-end rebuilt
Had new yokes installed in rear-end
Replaced shocks all around
Replaced steel 9-leaf with 360lb monospring
Replaced bushings related to suspension components in rear
Replaced cross member bushings
rebuilt power steering pump
replaced PS cylinder and position valve with rebuilt units
replaced plugs and plug wires
Added hi-temp sleeve over spark plug boots
replaced front wheel bearings
replaced brake pads all around
replaced front springs with 550lb coils
(Yes, I know it's not all relevant, but better too much info than not enough)

Diagnostically interesting:
Since the problem happens in all gears and seems to be based on the RPMs, it would seem to eliminate anything from the tranny back, as the RPMs of the drive train vary with road speed, not engine speed.

I've had this issue prior to any work being done on the car (the above list is not extensive), and nothing I've done thus far seems to have had any impact.

The motor mounts are good and were replaced in the last couple years when previous owner had the engine rebuilt. One bolt was loose and corrected during exhaust replacement (no doubt Bubba was on his game).

I don't know the condition of the tranny bushing.


I guess I'm stuck not really sure what to turn my attention to next, so I'm hoping someone has some experience with drive ability issues.

Thanks in advance.
Brian.

 

Brian-Is the trans a stick or auto? Probably doesn't matter, but check the tailshaft bushing, and check out the slip yoke for wear. Also check the trans mount, and it wouldn't hurt to have the driveshaft balanced, it may have thrown a counterweight

 

That's a really good question and darn obvious now!

The tranny is a 4spd manual, and a borg/warner, I believe (I have the codes and looked it up at one time :o )

The tailshaft bushing -- is that located at the back of the tranny where the slip yolk goes in?

What area on the slip-yolk is prone to wear and how can I best test for that (if the splines are worn, is there is a good way to test for play?)

Lastly, I have considered getting the drive shaft balanced, but I haven't had time to track down a place that can do it. I hadn't focused on that since I would have expected a driveshaft balance issue to manifest itself in the same manner as a tire being out of round/balance in that the vibration would be speed based, not RPM based. However, if I was so sure ...I wouldn't be asking for insight

Brian.

 

A worn out pilot bushing in the crank will also cause this vibration.

Does it vibrate in neutral? If so, you gotta consider the flywheel/clutch balance, also the engine fan/clutch can cause vibration.

 

Brian-The part about the vibes being RPM based seemed to have went right by me what caught my attention was the vibration seeming to come from the back of the car
That opens up a whole range of possibilities, possibly clutch related.
I'd still check the things I mentioned, especially since you went through the trouble to replace the other components.
BTW the tailshaft is the part of the trans the slip yoke slides into, there will be a seal and a bushing behind that. The slip yoke isn't normally a wear item, but it can happen(along where it makes contact with the bushing), especially if it's the original.

Hopefully others will see this and offer their suggestions

 

does the vibration change with clutch engagement vs disengagement? this would eliminate the driven components. worse under acceleration might indicate misalignment of the flywheel-driven plate contact. maybe something really strange like a loose flywheel bolt.

 

Good luck! I've got a 75 L-82 same problem I'm in the process of changing all the u's now ,waiting on a half shaft from van steel I sure hoped they shipped it today. Very few threads I've followed have found a cure I'm thinking ride it out and sooner or later it'll break cause you can sure throw a lot of money at it. Most research I've seen suggest a out of balance clutch assembly. My vibes are less noticable when I stab the clutch Do you run the stock damper after your engine rebuild?

 

All good suggestions. I would check the driveshaft for balance, bent. If the front support on the rear end is bad, you can get too much angle at that joint.

 

It's hard to tell with the clutch in/out due to the fact that RPMs drop so fast that I'd have to drive it up high enough that it would stay in the "range" long enough to know.

I also wish I had another one like it so I could compare some of the sounds the car makes with others to see what is "normal" and what is a wear sound.

I'm not ruling anything out, since Bubba had his hands on the thing, and some of the things I've found make me crazy (like a motor mount that was installed where the nut was nearly backed off the bolt! ) I did check the motor mounts to make sure they were good, and the engine only has a "reasonably small" amount of twist when I gun it.

I read your message wrong, as I was thinking you were saying 'harmonic balancer' ...which is easy to check. If it is a loose flywheel, I might not know any other way apart from dropping the tranny, right?

I can notice it more when the thing is loaded up, but it is still there on decel (clutch engaged), but it isn't as prevalent.

 

I was at my local Corvette meeting tonight and I was asking around for a local speed shop that can do a drive-shaft balance job. I think that is a reasonable thing to do and it's probably not too expensive. When I had the rear end rebuilt, I had the bracket out in the rear and nothing looked like there was damage or any bent sections to suggest it could be a problem. The old bushing was shot, so I did replace that with a fresh one -- that didn't make a difference.

I guess the only thing that got me hung up about the drive shaft is I wouldn't expect a vibration there to match RPM, but rather it should show up at certain speeds. The rate of rotation on the shaft would climb over the course of running through the gears and the problem seems to come in through all gears > 3500 RPM (assuming my dash tach is in the ballpark). This is why I figured drive and half shafts and tires were not a likely cause. I'm making a list all the same of things I can do ...I'm planning a lot of racing this coming year, so I'm spending the time to fix it now

Brian.

 

I just remembered when I fixed the tach that I was running the engine into the 3500 range and I don't recall any vibration, but I can run that test again to see if I notice it.

What test should I perform to find out of the fan clutch is out of "tolerance" for play? I did check that at one time, and it had some "wobble" in it but without knowing what is considered acceptable, I wasn't ready to flag it as bad yet.

Checking the clutch and flywheel is going to be a major task, but I'm keeping it on the list -- no reason to "trust" bubba's work on anything (even if he didn't). I do know Bubba rebuilt the clutch when the engine was rebuilt, but it was with the previous owner, so I have no idea about the specifics.

Is it normal to hear a "whirring" type noise when the vehicle is in neutral and the engine is running with the clutch engaged? If I disengage the clutch, I can hear the whirring spin down. It doesn't sound like grinding, but I don't recall other cars having a noise in this state, either.

Brian.

 

Sorry if my statements misled you. I do know that seat of your pants vibration is normally the back of the car, but I would think any chassis vibration could come through that method as well. Front suspension transmits through the steering wheel.

That's half the problem is that this is really hard to explain because all I have is what I've observed and it's not always easy to make detailed notes while hammering through the gears or going 70MPH

I have no reason to believe that work was done on the tranny at this point based on what I've seen thus far. I know the clutch was worked on as I have the documentation for that. I just lack experience troubleshooting clutch problems as all the other ones I've had didn't give me trouble.

Checking the yoke won't be too bad ...I'll just drop the front of the driveshaft down and see if there is any lateral play in it or if there is any visible wear. I've got the car's rear bumper completely removed, so it's not going anywhere for a bit (which is too bad is temps are 20 above normal -- so it's in the upper 40s)

Brian.

 

If it's not vibrating in neutral, then don't think it would be the clutch or flywheel.

Same with the engine fan. Amazingly, I remember somebody chasing a vibration and went through all the driveshafts, tires, etc only to find out it was a bent fan causing the vibration.

The whirring is probably just the throwout bearing but don't think that is causing the problem.

I would look at tire runout too. They can be perfectly balanced but out of round or have a slipped belt and they will vibrate something terrible. Would try a known good set of tires if I could.

 

I have a similar vibration in mine. It seems to be the worst pulling above 3500. I notice that stabbing the clutch cuts it in half. With the clutch in I can bring the RPMs back up and it will increase to about 75%.

I replaced the clutch many years ago and recall not finding any index marks at the time. I suppose if the cover was replaced they wouldn't be there anyway. My tires are 20 year old BFG T/A's. I have had the car big time sideways....OK and a little bit backwards a couple of times. I'm thinking they may have flat spots. My Spring project will be to replace the tires and pull the clutch and flywheel and either replace them with a balanced set or have them balanced. I'll let you know what I end up with.

 

how would you balance a clutch setup?

 

one more thing to consider in the clutch-flywheel alignment are the pins on the engine that slip into the bellhousing. i've never had use them, but i've read that offset pins are made to properly align the two. if the bell housing does not line up with the engine, even if your pilot bearing is centered with the engine off, under load the flywheel and driven plate will be off center. this obviously could cause your vibration that was rpm related. good luck, and keep us posted.

 

I'd have to buy a set of tires, as I only have what came on the car -- but they were nearly brand new when I got it. Of course, this doesn't exclude it, but I don't think there was much miles on there to get them out of round to that degree. Also, wouldn't that again be a speed related vibration and independent of actual engine rpms?

I have ordered a replacement fan clutch which I can install. The one there is likely original and is working, but may not be working in top form. There is some play when I flex the fan blades that include a 'thunk' as I flex it back and forth -- so maybe the bearing is shot in that. This, to me, is still easier than pulling the clutch.

I know these problems are a witch to track down ...but we have to build up the knowledge sooner or later

Brian.

 

C3nMe:
I have been following this since the first post yesterday and everyone has made some good suggestions.
Since we deal with driveline vibrations alot I am going to throw in my .02.
I am putting my money on the driveshaft being out of balance. Worse case scenario, driveline angles are off. Both scenarios will cause exactly what you are describing.
I do not believe it has anything to do with flywheel or clutch. Unless your sitting still and it is vibrating, than you can rule out those 2 possibilities. The rotating assembly will "shake" all of the time not just a certain speed or rpm.
Keep us posted.

Richard
Tech Support
Keisler Engineering

 

If you want to rule out the trans Bushing and slip yoke, Get the car up on Jack stands, Get under there and put your shoulder on the driveshaft close to the slip yoke and push up and down with alot of umph. you should see The slip yoke shaft move slightly in the bushing, there is a little clearance for trans fluid to lube it, but it shouldnt wobble or flop up and down. If it does, the slip yoke is worn out of round(oval), and the bushing is worn out also. Be sure to rotate the drive shaft 45 degrees and try again, That way you know youve seated the slip yokes shaft in all positions for checking.

 

Same thing for me before I pulled my L48 at 100K miles and installed a BBC 427. Its not on the road yet (next month or two) so I can't comment on if the vibration is still there.

To me a worn pilot bushing makes sense here. To my logic: A worn pilot bearing will allow the clutch to "wobble" from center-line when clamped. Engaging the clutch would "lock" the clutch disk off center-line. For me, at 3500 rpm it would become a pronounced vibration in any gear. Then stabbing the clutch lessened the vibration. Would it be that this "decoupled" the clutch such that not clamped by the pressure plate on its own rotating freely at the bottom of the slop in the worn pilot bearing?

I replaced my clutch with a centerforce II setup and a new pilot bearing on a GM 15lb flywheel. I'm hoping this will solve my issue..

-F