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vibration at 105km/ (63mph) only in 5th

Posted by nomis3613 
January 04, 2011 06:05AM
Hi Folks, I've got a strange intermittent problem that I'd much appreciate your thoughts on. Occasionally the car gets a bad vibration in 5th at about 105km/h. There isn't a noise, but I can feel it and looking through the rearview mirror is blurry so there's gotta be a lotta shakin going on.

It only happens at that speed with throttle on (goes away when coasting) and it doesn't happen at a) the same speed in other gears or b) the same revs in other gears. The car has previous had issues relating to frankenstein drivetrain mods, but allegedly this has all been fixed up and made kosher now.

Any ideas? Is there something in the gearbox that could cause slop only in 5th? (clutching at straws here...)

Thanks,
Simon
Driveshaft/guibo? Transmission mouunts?

alan
Quote
alanrw
Driveshaft/guibo?
That'd be my first guess. I haven't experienced this myself, but it has been widely reported in E30 Internetland that something as simple as putting a guibo back in with a different orientation can cause weird, hard-to-diagnose vibrations. The previous "frankenstein drivetrain mods" referred to strengthens my suspicion that something may be up with the guibo. If that's the case, I don't know if putting a new one in there would help or not, but it might be worth a shot.

__________
Dave
'91 325iX
In Theory, the car may be able to absorb/dampen certain vibration frequencies. This would, in theory, make it appear as if it was confined to a particular speed, when really you are just hitting a frequency that the car is completely unable to dampen. Blurry mirrors sounds like a drive shaft frequency to me, not something as slow as say a wheel.

I just went looking for a thread that doesn't seem to exist any more. I replaced my drive shaft a while ago, and did a lot of theorizing concerning vibration. My basic conclusion was that the drive shaft (depending on diff ratio) will be spinning 3 to 4 times faster. More often than not, this results in a movement that when you apply to a mirror, is faster than the human eye can deal with. I seem to remember that the vibration from an unbalanced wheel, or anything spinning at a similar rate, would not be enough to blur a mirror, only shake it. But that memory is 18 months old now and I can find the thread. Also, your personal vision will effect the result...as well as the definition of "blurry" smiling smiley

-----------------------------------------------------------------------------------
1989 - E30 - M20 - Manual. Approximately 270,000 miles
2000 - E46 - M52TUB28 - Manual. Approximately 110,000 miles

Alan, Dave, Earendil: thanks for your help. The guibo was replaced as part of the recent "un-Frankensteining", but yeah I guess that doesn't mean I should write it off.

The guibo was replaced to fix a previous driveshaft vibration problem, the new shaking definitely also feels like driveshaft vibration. But since that would be purely based on road speed, I'm wondering why it doesn't happen in 4th gear at the same speed.
Hmmm, good point, driveshaft speed would be the same in 4 or 5 at the same mph. Transmission mount that is activated by some frequency between the gear box and drive shaft?

alan
Quote
nomis3613
Alan, Dave, Earendil: thanks for your help. The guibo was replaced as part of the recent "un-Frankensteining", but yeah I guess that doesn't mean I should write it off.

The guibo was replaced to fix a previous driveshaft vibration problem, the new shaking definitely also feels like driveshaft vibration. But since that would be purely based on road speed, I'm wondering why it doesn't happen in 4th gear at the same speed.

Good point....
THEORETICALLY there could be a frequency generated by the transmission in 4th gear that cancels out the vibration caused by something spinning with the drive shaft. But now I'm really stretching theory smiling smiley

I always wanted to write an iPhone app that used the accelerometers to pick out vibration frequencies. Would be helpful in diagnosing vibration problems like this, or knowing how many cylinders are doing something "different". Oh well, maybe someday.

If you put the clutch in and rev the engine up high (while in gear) do you get a vibration?
With the clutch out and the transmission in neutral, and you rev the engine and hold it, do you get a vibration?
If you get to the point of vibration while at speed in 5th gear, does it go away if you put the clutch in?

I suppose the engine its self could be vibrating due to the stress placed on it? At any given wheel speed and incline, you would have two different kinds of stresses placed on the engine in 4th and in 5th. If it was a combination of RPM and engine stress, then it's plausible that you would only see it in 5th. Now, I'm not aware of an occurrence such as this, nor would I know how to explain why the engine would shake more at a given RPM+Stress. So you might want to possible ignore my last paragraph as me blowing smoke. And I just realized that the stress would change depending on the throttle, so I must be crazy. Why I don't delete this paragraph, I don't know. Perhaps I'm trying to give the illusion of being helpful.

-----------------------------------------------------------------------------------
1989 - E30 - M20 - Manual. Approximately 270,000 miles
2000 - E46 - M52TUB28 - Manual. Approximately 110,000 miles

Actually, your "blowing smoke" was helpful!

I'll try revving the engine at standstill like you suggest, but I doubt it'll cause vibration because it even goes away if I lift off the throttle.

The "engine stress" theory might be onto something. The problem even happens at light throttle so I doubt it is the engine struggling, but yeah it could be related to the engine somehow.

Surprisingly, I have access to very precise vibration measuring equipment! So if I can consistently reproduce the fault, then I could actually post the spectrum of vibration...
Fifth gear is an over drive that uses both internal shafts of the transmission to increase the output speed. Power is transferred from the upper shaft to the lower shaft and back to the upper shaft. There could be some malfunction within the transmission. It might be good to investigate with an oil change and exploration with a magnet in the oil and inside the transmission case to see if an excess of steel particles can be found that might indicate a badly worn gear throwing chunks of metal into the others.

Bob in Everett
rkj
Quote
nomis3613
Hi Folks, I've got a strange intermittent problem that I'd much appreciate your thoughts on. Occasionally the car gets a bad vibration in 5th at about 105km/h. There isn't a noise, but I can feel it and looking through the rearview mirror is blurry so there's gotta be a lotta shakin going on.

It only happens at that speed with throttle on (goes away when coasting) and it doesn't happen at a) the same speed in other gears or b) the same revs in other gears. The car has previous had issues relating to frankenstein drivetrain mods, but allegedly this has all been fixed up and made kosher now.

Any ideas? Is there something in the gearbox that could cause slop only in 5th? (clutching at straws here...)

Thanks,
Simon

So, Simon. If you're going along in 5th gear and it's vibrating away and you push the clutch in and it stops?, or you let off the gas and it smooth's out too?

Rick
Damn intermittent problems! Today, nothing I did could produce the vibration. Just in case, I tried the suggestions of revving it with the clutch in etc, but also nothing. I guess I'll swap the fluid and check for metal.

Quote
rkj
So, Simon. If you're going along in 5th gear and it's vibrating away and you push the clutch in and it stops?, or you let off the gas and it smooth's out too?
Goes away if I reduce the throttle. I haven't been able to test out engaging the clutch yet, but my guess is that it will also go away, because it seems to be caused by torque.
Okay, if I were you I would look VERY carefully at your U-Joint. Mine went bad last year and caused intermittent vibration. Why? Because the metal was starting to tear. Sometimes hard acceleration would knock it into the torn notches, causing and unbalanced state. During engine braking the pressure would knock it back into it's "normal" spot, causing the drive shaft to balance out and the vibration to vanish. When looking at it from 12 inches and a bright flashlight, the U-Joint was obviously bad.

-----------------------------------------------------------------------------------
1989 - E30 - M20 - Manual. Approximately 270,000 miles
2000 - E46 - M52TUB28 - Manual. Approximately 110,000 miles

rkj
Quote
nomis3613
Damn intermittent problems! Today, nothing I did could produce the vibration. Just in case, I tried the suggestions of revving it with the clutch in etc, but also nothing. I guess I'll swap the fluid and check for metal.

Quote
rkj
So, Simon. If you're going along in 5th gear and it's vibrating away and you push the clutch in and it stops?, or you let off the gas and it smooth's out too?
Goes away if I reduce the throttle. I haven't been able to test out engaging the clutch yet, but my guess is that it will also go away, because it seems to be caused by torque.

When my driveshaft got bad enough to cause vibration it was still seemed tight, I had to have someone rock the car in gear while I was under with my hand on the U joint; Then I could feel it without question. I ran the rear of the car up on 2x10's to give me enough room...

Rick
January 14, 2011 06:16PM
I went to buy replacement gearbox oil for the Getrag 260 and they only had non-synth GL4 oil where I went. The box is currently running fully-synth GL5/6 (Penrite SIN 75W90).
1) Is it ok to go back from synth to non-syth (I've heard this is a bad idea for engines)?
2) Do you reckon this oil will make shifting any worse?
3) If I do find it worse and want to go back to synth is that ok, or is it bad to swap back and forth?

Rick, thanks for the U-joint suggestion. I'll give it a try when I'm swapping the gearbox oil.

Thanks!
January 14, 2011 07:47PM
Quote
nomis3613
I went to buy replacement gearbox oil for the Getrag 260 and they only had non-synth GL4 oil where I went. The box is currently running fully-synth GL5/6 (Penrite SIN 75W90).
1) Is it ok to go back from synth to non-syth (I've heard this is a bad idea for engines)?
2) Do you reckon this oil will make shifting any worse?
3) If I do find it worse and want to go back to synth is that ok, or is it bad to swap back and forth?

Rick, thanks for the U-joint suggestion. I'll give it a try when I'm swapping the gearbox oil.

Thanks!

Don't worry about mixing the lubricants so long as they meet the specs for the car. You can generally mix these with impunity; just don't mix hypoid with non-hypoid types.
January 14, 2011 10:26PM
The seals on my transmission started leaking when I went to synthetic. Not sure what they would do if I went back to basic.

Do be sure and inspect all the drive line parts. The Guibo and u-joints and also the center bearing. It might be the rubber mounting of the center bearing is worn out and allowing the shaft to bang around.

Bob in Everett
February 28, 2011 01:17AM
Update: vibration gone, no idea why!!

Fingers crossed, but I just came back from a 600km weekend away and the driveshaft stayed nicely vibration-free. Before the trip, I had the mechanic replace the selector and output seals (I thought just the output seal was leaking, turns out the main culprit was the selector seal) but I highly doubt that had anything to do with it.

Thanks for the help along the way.
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