Vibration.... help

shlim · 12-12-2001, 12:03 AM

Hi,

while I was doing a fast corner, I suddenly felt strong vibrations from the clutch pedal and gearshift. Pulled to the side and check to make sure that all the belts, liquids, mounts etc were alright. Looked alright but now I feel strong vibrations especially from about 1500rpm and up. Otherwise the engine seems to run fine, although sounded a little louder and the idle seems to be slightly higher.

Looked at the engine mounts but those seem okay.

Is there anyway of testing the mounts ? Can it be something else ?

shlim

Lawrence Coppari · 12-12-2001, 04:02 AM

If it vibrates with the car NOT in motion, it probably is either the motor mounts or the balance shaft jumped a tooth. The balance shaft belt skipping a tooth gives a higher frequency vibration. A bad motor mount will vibrate at low rpm. If it is a timing belt, then belt is loose. If it were to break, it could get wound up in cam belt causing expensive damage.

shlim · 12-12-2001, 04:47 PM

The vibration is there, regardless of whether the car is in motion. It's more of a high frequency vibration - the gearshift and clutch pedal actually feels like one of those battery operated massagers. (the seat becomes one for a brief period at about 2000+ rpm then gets drowned with all the vibration coming in from the body shell).

Strangely enough, the engine note has also become more aggressive and loud.

I had a independent mechanic (they're more focused on 911) take a look and he thinks it's the gearbox mounting as the vibration seems to be stronger from the gearshift and clutch pedal, but if that's true, will there be increased vibration when revving at a standstill ? I though the gear box would not have moving parts with the gears disengaged.

Frankly, the engine mounting looks okay to me - very little twist when the engine is revved hard. There is also no liquid leakage as far as I can tell but then it was raining when the dreaded vibration started...

Any ideas ?

shlim
85.5 944

drew1 · 12-12-2001, 05:58 PM

shlim,
The mechanic might be right since even with the clutch in & gears not turning, vibrations can travel by the rigid torque tube. If trans is loose the muffler thats hanging from it could be pulled loose making the added noise.

I think 944 motor mnounts are checked by measurement. Manuals give this figure.

drew1

DennisR_944 · 12-13-2001, 01:26 AM

Try this Tech Article on your motor mounts. It'll tell you how to check them.

tifosiman · tifosiman's Garage

shlim,

I just went through this on my 85.5 N/A. I can say with 99% confidence that it is a balance shaft belt issue. Have that checked immediately, don't drive the car very much until you get it checked out. Mine actually snapped, and luckily it just wrapped itself up in the corner of the cover. Sometimes when they snap they wrap themselves around the timing belt, causing damage to it and fast failure, and we all know what that leads to...........

Tifosiman

Lawrence Coppari · 12-13-2001, 04:02 AM

The hand massager description sounds like a balance shaft issue to me. Those shafts turn at 2X crank speed. Better get it looked at because of what can happen.

When changing belts, I once did not get the balance shaft oriented correctly. It was a high frequence vibration as you describe.

shlim · 12-13-2001, 06:13 PM

Hmmm,

l better get the balance shaft belts looked at immediately. I'm still using the car on a daily basis.... playing with fire ?!!

I would guess that it's the belt jumping a tooth... the car starts up with minimal vibration but only acts up once I rev. The gearbox was removed twice in the last few months - and the engine was rebuilt last year. I'm sure the mounts have been looked at (although no assurance that it'll suddenly expire....).

If the balance shaft belt skips a tooth - does it historically fail soon after that ??

I'm very concerned now ! I still need to use the car for the next couple of days !

shlim

joegeidl · 12-14-2001, 12:08 AM

See posting by "drew1" dated 9-17-00 for information on lining up balance shafts.
This is the best info on this subject I have found!
Thanks again Drew.

tifosiman · tifosiman's Garage

It probably has done more than skip a tooth, it has probably sheared a few off of the belt and is in a permanent state of mis-alignment. This isn't so noticable at low rev's, but between 1500 and 3500 it should become noticable. Driving it is playing with fire. I took a good look at my belt when it was removed, and there were 4 or 5 teeth sheered off, and you could see where the belt failed at the end of that section where there was rub and wear where the slippage continually occurred. I was just very lucky. This probably only took a very short while, as the car was only driven about 50 miles from the time the vibration started (skipped/sheared teeth) and the time it arrived at the shop for the repair. It completely broke sometime during that 50 mile span. Be very careful.

Tifosiman

nib5 · 12-15-2001, 04:22 PM

Get the top front cover (black plastic) off and I can almost guarantee a broken balance shaft belt will be lurking behind it. If it is broken, extract it and check it against a new one, some pieces of it may have been pulled up into the ally cover behind the distributor - though I suspect you might wish to get both belts done at the same time (wise).

shlim · 12-17-2001, 05:02 PM

Just got back from my short holiday. Thanks for all the post and suggestions.

There really isn't much space to work on the belts is there ? Didn't get the time to open up the cover yet, but will plan on doing that soon. The car is laid up in the garage now, I'm not going to drive it till, I'm sure everything is fine.

Looking back, I did find that the car had more whinning/swishing noise from the belts or bearing/tensioner than most of my friends' 944. This was since Jan '01. It might have been improperly tensioned. The whinning/swishing noise does seem quieter now ! But not completely gone.

It's a good thing the problem started when I'm back home, I took the car up for a 800km trip up north during my last business trip, just a few weeks ago. It would have been pretty difficult to tow the car to the nearest Porsche workshop.

Will update all of you on what I find.

shlim
85.5 944

shlim · 12-17-2001, 11:48 PM

Had the belts checked and MAN ! you guys are GOOD !

The balancer shaft belt is broken. I guess I was lucky that the belt did not wrap itself around the timing belt or it'll be overhaul #2 ! I'm surprised that the vibration is not more without the balance shafts. Frankly, I was just expecting an out of sync belt.

The car actually has noticeably more poke (low end) without the balance shaft. Especially apparent when on that economy, long geared 5th.

Unfortunately, I think this job is beyond me and I don't have a tensioner guage - guess it's back to the Porsche dealer. Ouch, this is going to hurt.

How long does a typical water pump last ? I'm wondering if I should change the water pump at the same time. I suspect mine is about 2 years old (daily used car only for the past year). I'm definitely changing the timing belt and tensioner - that's not worth stinging on !

Mystery solved (not fixed though...), thanks guys !

shlim
85.5 944

Lawrence Coppari · 12-18-2001, 03:03 AM

Unless the water pump has 60-70K miles on it, I would not bother changing it. But the problem with water pumps is that some live to a ripe old age, others die young. To a large extent it is a crap shoot. Place your bet and take your chances. Do whatever eases your mind and suits your wallet as well.

With these cars, it definitely pays to learn how to service them yourself. The savings is astronomical.