Significant Vibration right at 4000rpm - Is this bad motor mounts?

[phoenix_iii]

Had some work done on my newly acquired 85.5, I've put less than 200 miles on it since owning it (mostly to transport it). Anyway, car drives well now (tbelt,pump,balance, etc), but I get really bad vibration (hear it and really *feel* it right in the shift knob) right at 3900-4050 rpm. Just that range, then feels smooth from then on, or before it.

I've just been shifting at 3500 or so to keep from repeating this condition. The mechanic (place specializes in foreign cars, has 911, 944, Mercs and bmws as customers) said I (forget the language of it) either 'might want to' or 'should' replace the motor mounts. I already have new mounts on the way and will have them install it... but I wanted to make sure they would address that vibration issue.

Any thoughts/other things to check?

It also does this with the car NOT in gear (in neutral), so I do not think it's anything with the drivetrain.

Thank you for your input!


*update*
Symptom update

In neutral:

Clutch pedal upwards (not depressed) - some vibration
Clutch pedal downwards (to the floor) - significantly more vibration, 3-4x

On cold startup this morning (First cold morning I've had the car), the idle had some vibration, so I think the mechanic was right that the motor mounts could stand to be replaced (already ordered/searched for best options). Thanks Pelican

[Paul_Heery]

How old is the clutch? Is it an older rubber-centered one?

A vibration at those RPMs could be indicative of the rubber disintegrating.

[zedsn]

My 83 944 had a bad vibration that I could feel through the shifter at 4 grand or above and upon further inspection the person that worked on the 944 before I owned it had the timing and balance shaft belts off a tooth each. Maybe check your balance shaft belt and make sure it is correct.

[phoenix_iii]

Not sure of the clutch...

But I mean, it's *really* just like 3900-4100... 4300->5500 is smooth. I just don't like breaching the 4k mark to get there, but the times I was doing it to test it was just fine.

Clutch feels good though, no indication of slipping, and car was converted to r134a and interior in excellent shape, so it would appear maintenance was done...

They did the timing/balance shaft, but they also kept my spare door handle (in the car) by accident, so I will need to stop by to get that back (also going to have them do the motor mounts since I am sans a place to work)

[zedsn]

As far as I know motor mount vibration is most noticable at idle and not at strictly 4 thousand rpms.

[phoenix_iii]

Originally Posted by zedsn

As far as I know motor mount vibration is most noticable at idle and not at strictly 4 thousand rpms.

Yeah... makes me think/wonder balance related things... but wouldn't it be over the whole (or 3k up) rpm range?

[zedsn]

yes, mine was 4 grand to redline. You are correct there. You may have another issue not covered here already?

[John_AZ]

Motor mounts typically show at idle 950-1100. I think it may be the original? rubber clutch disk. Item 9.4 and your car may not be the typical +-3000rpm.
http://www.connact.com/~kgross/FAQ/944faq09.html

John_AZ

[phoenix_iii]

"With age or abuse, the rubber center in normally-aspirated cars' friction disk can deteriorate then break. When it has deteriorated, chunks of rubber can separate and sometimes lodge themselves between the disk and the flywheel, inhibiting normal clutching. The missing rubber can also produce a noticable drivetrain vibration, typically aroung 3000 RPM."

Hmm... nothing is inhibiting normal clutching though, thank goodnees!

=)

Also, it happens with the car in neutral, and foot off the clutch. I wonder if I should test it with the pedal to the floor, and neutral?

p3

(edit) It does say 'sometimes' inhibits normal clutching... ?

[John_AZ]

If the past owner had the disk and pressure plate changed and/or did the rear engine seal, the step sometimes skipped is to balance the pp/disk AND the flywheel. This would cause a vibration as well.

John_AZ

[phoenix_iii]

Originally Posted by John_AZ

If the past owner had the disk and pressure plate changed and/or did the rear engine seal, the step sometimes skipped is to balance the pp/disk AND the flywheel. This would cause a vibration as well.

John_AZ

Would this not be similar to an unbalanced wheel? Where it's not noticable below say 30mph, but at 50mph and HIGHER it just gets worse, the wobble (unbalanced) becoming more pronounced?

Just trying to understand, thank you for the intelligent posts!

[John_AZ]

Originally Posted by phoenix_iii

Had some work done on my newly acquired 85.5, I've put less than 200 miles on it since owning it (mostly to transport it). Anyway, car drives well now (tbelt,pump,balance, etc), but I get really bad vibration (hear it and really *feel* it right in the shift knob) right at 3900-4050 rpm. Just that range, then feels smooth from then on, or before it.

It also does this with the car NOT in gear (in neutral), so I do not think it's anything with the drivetrain.

Maybe similar to out of balance wheel but not to the extent of more vibration at higher speed or rpm. A poor rubber clutch disk usually vibrates at a specific rpm and may not be vibrating at a lower or higher rpm.

"Hear it and really *feel* it right in the shift knob" again for me points to the clutch out of balance.

I do not think it is the drive shaft bearings, but maybe with the transmission in neutral and engine at 3900-4050 rpm, you could test if the vibration occurs with and without the clutch engaged. Please be careful...set your emergency brake.

John_AZ

[robm-951]

I would double check the balance shafts and especially the pulley orientations on the shafts. It's a common mistake to install the lower pulley in the wrong keyslot. Check out the picture in the second post of this thread:

944 balance shaft setup

Notice the keyslots shown in both the upper and lower pulleys and which slots the balance shaft keys go in (filled in black on drawing).

[Cking1975]

If your engine revs up in neutral and is fine, then there is something wrong with your transmission.

[phoenix_iii]

Originally Posted by Cking1975

If your engine revs up in neutral and is fine, then there is something wrong with your transmission.

It does NOT rev vibration free at the 4k mark in neutral. I want to try it with the clutch pedal down, and up, to see if there is a difference, but I tried it one way (I forget which), in neutral, and had the vibs!

p3


However, the 'intensity' of the problem was diminished the last few times I pushed into 4k (and then past, which was smooth)

[firefighter7128]

does your car have a "turbo sound" to it? i have the same issue. the wine sounds bad ass, but i dont think its supposed to have that sound.

[rmconner80]

Originally Posted by John_AZ

If the past owner had the disk and pressure plate changed and/or did the rear engine seal, the step sometimes skipped is to balance the pp/disk AND the flywheel. This would cause a vibration as well.

John_AZ

+1 I just did this and I can attest it adds some vibration. Thought I had read up on everything I need to do / replace, used the FSM, Clark's, Haynes.... but failed to have the assembly balanced when I got the flywheel resurfaced. It adds some vibration. Bummer. Too late now, I'm not taking it apart again!

[phoenix_iii]

Symptom update

In neutral:

Clutch pedal upwards (not depressed) - some vibration
Clutch pedal downwards (to the floor) - significantly more vibration, 3-4x

On cold startup this morning (First cold morning I've had the car), the idle had some vibration, so I think the mechanic was right that the motor mounts could stand to be replaced (already ordered/searched for best options). Thanks Pelican

[Z06C5R]

Same exact problem here... The PO had both the timing belt and the clutch replaced by a Porsche mechanic, so perhaps either could be a problem source?

Can anyone point me to some info on re-aligning the balance shafts? Most of the stuff I've read only covers how to check if their out of alignment...

[phoenix_iii]

Originally Posted by Z06C5R

Same exact problem here... The PO had both the timing belt and the clutch replaced by a Porsche mechanic, so perhaps either could be a problem source?

Can anyone point me to some info on re-aligning the balance shafts? Most of the stuff I've read only covers how to check if their out of alignment...

I would be thinking that if the P mechanic did it would not be suspect... I need to call them today anyway and I will see if I can talk to my guy.

Also, if its doing it with the pedal down (clutch 'out), then I would think it's an engine/balance/mount issue, though not sure why my linkage/shifter would get the brunt of it, but maybe it's just sensitive. =)