Jim Chambers

I asked this question a few weeks ago but it was combined with another subject (timing belt tension, now fixed). It is time to attack the vibration.

Symptom: Car drives fine, no unusual drive train vibration until I wind it up. When I reach about 5000 rpm in any gear the shift lever (5 spd) begins a strong and loud vibration. I cannot come close to holding it still by hand. I seem to both feel and hear the extreme vibration. I suspect that it is a drive train issue, not just shift linkage.
Where should I look first?

Motor mounts? They do not appear to be collapsed. Is there a visual or measurement test? I have yet to see if the engine rocks on start up.

Trans mounts? Not sure how to evaluate them.

Torque tube bearings? How do I check? (this repair scares me as much as motor mount replacement).

Other?

300+ horsepower at 5000 rpm is great fun. I want to go there!

Stromius

Vote is for torque tube bearings. See great replacement write up on Sharkskin's site to update to new ones from Constantine. Starts here: http://members.rennlist.com/sharkski...TorqueTube.htm

Bonus is it involves use KY and profolactics

Mrmerlin

a quick check of the MMs is to look at the bottom of the oil pan if its below the crossmember then the mounts are worn/ failed
Usually replacing the trans mounts has cured a vibration like this but the MMs also were done

Lizard928

Jim,

I still think that you should inspect that the TT bearing carriers have not migrated.

Actually thinking about it. There is a bit of a convoy heading up this way soon.
Tom Dingwall from your area, Ed Boyd, and a few others are coming up for a rack party. You could join them and we could verify as to what the problem is.

SeanR

Easier test for the motor mounts is to open the hood, get in the car and start it. The engine should rock then, and when you hit the gas.

Black Sea RD

It sure does sound as if the TT bearing units have either moved from their assigned positions or the bearing inserts have fragged and are no longer holding onto the drive shaft well.

Bad motor mounts tend to send vibrations through the car and can be felt more in the seat of your pants.

First bearing units we would check is the front one after you remove the flywheel cover. It shouldn't be more than 10-12" in. More than that and it sets up for vibrations to start happening.

Do you hear a clunk or thud under your 928 when braking or getting on the gas abruptly? The TT dampener has been known to come apart and start sliding up and down the TT moving the bearing units around too.

HTH,

Jim Chambers

I will check the bearing location in the next couple of days. No thud on braking or sudden acceleration that I have noticed but I will double check. Everthing goes from "just fine" to terrible at about 5000 rpm. I notice nothing unusual until then. Thanks

Jim M.

If it's RPM related and not speed related I can almost guarantee it's torque tube bearings that have shifted. Mine used to do it at 4000 RPM.

atb

rev w/clutch in with no vibration means its not the torque tube.

dprantl

I'm having the same issue. In neutral with my foot off the clutch, I have a bad vibration at 5k RPM. Rev the car up with clutch depressed and no vibration all the way past 6.5k RPM. This rules out motor mounts 100%. I thought maybe it was the clutch so I did that job a few weeks ago. Arm bushing was dead and was replaced and the guide tube bolts were loose. Pilot bearing was also freewheeling, so it was changed out. After buttoning everything back up with new discs, PP and release bearing, vibration is still there. So much so, that it would loosen the guide tube bolts. In my mind it is definitely the torque tube which is what I will be replacing soon. In the interim, I have put slightly longer guide tube bolts with nylock locking nuts on the other side of the bolts.

Dan
'91 928GT S/C 475hp/460lb.ft

Jim Chambers

I have done a little more "testing". Everything starts exactly at 5000 rpm. Nothing gradual ,it's like turning on a switch. The shift lever goes crazy, shakes and rattles loudly. Kinda scary at first. Hard to concentrate because I don't like to keep it there very long, but I don't really feel anything in the "seat of the pants".
Tried 5k sitting still in neutral. It again starts at 5K and there is only a slight difference with clutch in or out.
Does the above info. give anyone a better idea of what I am dealing with?

Jim Chambers

OK, now I'm wondering about motor mounts. Had my wife start the engine while I watched. No movement. I worked the throttle a bit by hand, no movement. Can anyone tell from this pic if the pan is too low? I am not sure what to look for.

Landseer

Looks low. How about a picture of the oil filler neck relative to the cross brace?

the flyin' scotsman

Can't tell from the pic but from the descriptions it reads as if the motor mounts are gone.

Blip the throttle by hand a few times in quick sucession the motor should rock, if not MMs are toast.

Jim Chambers

Here are 3. Tried to take them straight on rather than from above.