LTCMontana

My 95 ad to sit for almost a year due to other 'shinny' objects in my shop. Now that is running, I get a very high frequency vibration at around 3400 rpm.

The balance shafts have been checked and all is good there.

Could this be motor mounts?

Boeing 717

In my case I have big vibration in the lower rpm range, below 1000. I would think that at higher rpms the engine would be running smooth enough that motor mount condition wouldn't be as noticeable.

968workaholic

Normally your description of the rpms that the vibration is happening would suggest a balance shaft timing issue. As stated above, motor mount vibration is felt most at idle

biosurfer1

Wish you could sit in my car, then it would be clear. Terrible vibration at idle which goes away with any increase above idle. Driving the car is perfectly fine, but the annoying shaking at idle is the clear indication they are shot.

Boeing 717

Are you going to change them out yourself?

biosurfer1

most likely yes, paying labor on top of the crazy cost of the mounts just seems....crazy.

oppositelock

Worn out engine mounts feel like an out of sync balance shaft belt. So if BSB is timed, then will be engine mounts. The mounts in these cars are a consumable part that needs periodic replacement. Strongly suggest OEM Porsche mounts. Too many issues with aftermarket mounts.

RajDatta

You could be out on the balance shafts even though they look right. Was anyone in there to change the bs seals? Often, people get the pulleys wrong, so the 3400 vibration seems to suggest one of the bs out of sync.

Fritz968

Last year I replaced my bad engine mounts with an aftermarket brand. Last week (and less than a year) they needed replacing again. Lots of vibration at idle. This time I paid the premium for original Porsche parts and the difference is night and day. Huge improvement, and hopefully they will last longer.

I also had an additional bad vibration at about 3k rpm, particularly when the car was under load, accelerating up a hill etc. Unfortunately new engine mounts didn't cure this. The vibration is less severe now, so my bad engine mounts was amplifying whatever the real issue is.

On a side note: I've been reading this forum for about a year and finally have some relevant experience to be able to contribute.

biosurfer1

Did you DIY the mounts? How'd it go?

Fritz968

No I had my mechanic do them as I needed his help to figure out the initial cause of the vibration. He charged two hours labour for the job

LTCMontana

UPDATE: Went back into balance Shaft cover to look again. Shafts were timed right but the belt was loose. Vibration was from a harmonic on the long side of the belt at ~3050 RPM. Retightened the belt and all is well.

I ran the car with small cover off and was amazed to watch the balance shaft belt work through different harmonics as the revs changed.

RajDatta

Love cheap fixes