Mark R.

My 88 five speed has a strange vibration which peaks at 1500 RPM.
It idles totally smooth, has great power, and is smooth at higher RPM's.
But when I transition through the 1500 rpm range, it is surprisingly rough.
I feel it through the steering wheel, driver's seat, and shifter handle.

I recently checked the compression and changed all the spark plugs.
The vibration was there before that (BTW, all cylinders were 180-190).
Total top end refresh (IM, cam covers, all components) earlier this year.
(after changing the plugs, I had ZERO dirt on my hands. What a pleasure!).

The bottom end is a different story: It is due for OPG and engine mounts.
I jacked up the engine to see how much the vibration is related to the mounts.
It did diminish it some, but vibration is still clearly there, at the same RPM.

The vibration is absolutely unchanged whether the clutch is in or out.
So I am ignoring the torque tube, and focusing on the engine and clutch.

So, finally, my question: How do I check the clutch for any imbalance..?
In particular the pressure plate, since that is what spins with the clutch in.
I searched but could not find anything directly related to this specific topic...
Is there a procedure I can follow to determine if everything is okay there?
Thanks!

.

BRB-83-911SC

Hi Mark - see the thread attached. On my car, we chased down many rat holes, including balancing the clutch pack, before resolving with new motor mounts. I'm not saying this is your issue, but you can read through and see what we did in the process of chasing this down. Good luck.

https://rennlist.com/forums/showthre...ferrerid=34007

MFranke

Sounds like the motor mounts

Mrmerlin

before you chase anything, replace the pan gasket to prevent leaks on the garage floor,
WYAI swap in a new set of MMs. report back your findings

Mark R.

Yeah. I know, I know. I should do the mounts before I do anything else.
That was already planned. Just not right away in the immediate future...

Brian - how did you balance the clutch pack? Is there a procedure somewhere?
I'm just curious about what's involved, to possibly check and eliminate that.

Thanks guys.

BRB-83-911SC

I took it to a machine shop that does balancing work. They balanced the PP, IP and Flywheel (dual disk clutch), properly indexed, as a unit. It tested fine as is below 7,000 RPMs - so that wasn't my problem.

Kuba

I have the same vibration on my '87 5mt with 80k mi.

It vibrates the same clutch in or out but the vibration is reduced when the car is rolling in any gear at 1500rpm and reduced to almost nothing with more engine load in gear (accelerating through 1500rpm).

I've changed the air filter and spark plugs, no improvement.

Changed the engine mounts and it is reduced to non existent when rolling in any gear at 1500rpm.

It still vibrates very lightly stopped, revving at constant 1500rpm.

I thought about changing transmission mounts but they are not collapsed yet and I can live with the current vibration.

It seems like driveline load helps reduce the vibration and good engine mounts help mask it. Is the engine naturally imbalanced at 1500rpm?

928NOOBIE

The harmonic balancers are made out of rubber..I'm having some similar issues with my 86 5 sp...it will do some wierd vibration around 1500 then goes away..but isn't severe. We are replacing the harmonic balancer this winter. New clutch was installed by a 928 guy who still had a contact that would balance the clutch assembly for him so that was also done...maybe its the harmonic balancer?

Mark R.

Yeah. I was afraid the process would involve removing the assembly and taking it to someone.

It very well may be. But either way, it seems to be the speed that accentuates the bad motor mounts the worst.

I really don't know, but it seems like that would be felt at other additional RPM's.

Yes. I know for sure the mounts are shot. So I suppose I should change them before I waste my time chasing anything else.

The floor is clean. No oil ever makes it that far. Just a small amount of seeping that kind of blows back onto the underside.

In fact, I'm actually considering doing JUST the motor mounts. The vibration really annoys me, and the oil seepage does not.

I realize the OPG is a major WYAIT item to do with the mounts, but it will significantly magnify the time and effort of the MM job.

I am just extremely tight on my available time at the moment, and I can do the mounts in one afternoon.

I can get in and out with the MM's and comeback later when I have more time to spend doing the OPG.

So whattayathink..? Is that crazy..?

SteveG

MM are a dirty, heavy lifting job. It makes sense to do gasket while you have the rack, etc. disconnected. But, YMMV, if the pan seepage is that minor, let it go, you seem to have it under control.

Mrmerlin

If it was me I would swap out the pan gasket with the cross member out of the way,
this way you get to clean the pan properly and inspect the innards of the block,

time saved by not doing the OPG while the MMs are also done .. I just dont see it.

I would rather get dirty once then have to get dirty twice while working on the same area

Wisconsin Joe

Well, to counterpoint a bit...

You will be doing a ton of cleaning when you do the MMs. You're talking about 20+ years of built up crud.

That crud won't be back in a year or so when you go back in for the OPG (unless the gasket is leaking pretty bad).

There's a pretty steep learning curve when you do the MMs. I'm going to guess that, having done it, I could duplicate the job in about 2/3 the time. Simply because I won't have anywhere near as much to figure out. Still nowhere near as fast as MrMerlin or Sean could do it, but a lot faster than the first time.
And adding the OPG to the MMs adds a fair amount of time to the job.

So it's not completely crazy to want to skip the OPG for now.

soontobered84

I would also do the OPG whilst I was there. If time is a factor, ask a buddy(or more than one buddy) to help. They don't have to know anything but tools. The time factor getting out from under the car, getting the correct tool, and getting back under the car and into position is a major time killer. Having someone there just to hand tools to you and maybe clean parts saves major, major time. Having someone there to help that has actually changed MM and a pan gasket and can wrench alongside you ensures that it could all be done in an afternoon!

MFranke

The OPG gasket is an extra half day, if that, mostly spent cleaning if you want to clean pan and everything else up nicely. Good idea to do. I would reserve a long weekend, plan it out, and go for it.

The MMs take the longest. Pay good attention to the orientation of the hardware as you remove them (take pics) to save time figuring it out later. Be prepared with extra 13mm bolts and nuts for the MM mounts in case you either strip the alum block thread or old bolt breaks. The new bolts need to be long enough to allow a nut on top. It only takes one bolt issue to slow you down if you are not prepared. Easy fix and basically little to no delay if you have reverse bit and extra hardware.

Mark R.

Of course, if I decide to do just the mounts, for now, the obvious next dilemma is:

-> Should I tighten the OP bolts with the existing OPG, or just leave them alone..?

I worry I won't be able to resist checking them, and snugging them up some...