Vibration Issue: 1988 924S

[Davie Boy]

Hey all,

I recently had some major work done on my car, including engine swap (used '88 2.5 liter), new clutch, and cheapy motor mounts. The car is all back together now, but I'm feeling some vibration. The funny thing is, the vibration is cyclical and not constant. If I have the car in neutral and hold the engine speed at a constant RPM, I can feel the vibration slowly go from smooth to rough and back again, over and over again. The mechanic I took it to who isn't super familiar with these cars thinks it's most likely the cheapy motor mounts. I'm not a mechanic, but in my mind - if the issue was simply cheap motor mounts - it seems like the vibration would be more constant/linear with the rise and fall of engine speed. My initial thought was the an out-of-balance flywheel. Maybe it wasn't resurfaced correctly? Of maybe an issue with the balancer shaft?

Does this issue sound familiar to anyone?

[9FF]

You can feel motor mounts mostly at idle, sounds like your balance shafts are out and the most common mistake with those is the lower shaft being set 180 degrees out. It’s an easy mistake to make for someone unfamiliar with these cars. Does it get any better in the higher rev range?

[Ga 951]

+1 on the balance shafts. I had one that was a tooth off. It made this harmonic vibration in the interior that drove me crazy.

[Davie Boy]

Originally Posted by 9FF

You can feel motor mounts mostly at idle, sounds like your balance shafts are out and the most common mistake with those is the lower shaft being set 180 degrees out. It’s an easy mistake to make for someone unfamiliar with these cars. Does it get any better in the higher rev range?

No, it doesn't get any better with higher revs. The rate at which it moves from smooth to rough and back again is quicker though, as you might expect.

[Davie Boy]

Originally Posted by Ga 951

+1 on the balance shafts. I had one that was a tooth off. It made this harmonic vibration in the interior that drove me crazy.

Yeah, this makes more sense to me than an issue with the motor mounts.

[Davie Boy]

Ok, so I have more information. I was finally able to put a good amount of miles on the car and get a better feel for what's going on. The vibration is most intense in 1st and 2nd gear, and at higher RPMs. While still noticeable, it's not as bad in higher gears, even at higher RPMs. I also noticed that after the car gets warmed up and has been driven a few miles, the vibration overall diminishes somewhat. It's definitely still noticeable, but it's not as intense. Does this still sound like a balancer shaft issue?

[irish44j]

Cheap motor mounts are only rough at lower idle (I have the cheap ones on my '88 924S as well, and even with my idle bumped up to about 1050rpms, they're smooth....but rough below that). But above idle the motor mounts wouldn't cause vibration to any notable extent.

I'd still lean toward balance shaft being off a tooth or more. Does it do it all the time, whether the car is moving or not?

[Jfrahm]

This sounds like a balance shaft issue and you should check the balance shaft phasing. If you cannot do this yourself you are going to go broke owning the car so it'd be good to start learning that sort of thing. It's a bad idea to run with the vibration as it can cause cracks in the oil pickup tube and that leads to oil starvation. The belt tension needs to be checked and re-set also after the belts run-in.

Re: engine mounts, How cheap is cheap? I put in the Meyle mounts and they are working great, very smooth idle. It appears that the URO mounts are Meyle with the name ground off, but that might just be the mold and not the rubber compound and filling. My concern about the Meyle mounts is that they may not last 20 years but function-wise they are fine.

[Davie Boy]

Originally Posted by irish44j

Cheap motor mounts are only rough at lower idle (I have the cheap ones on my '88 924S as well, and even with my idle bumped up to about 1050rpms, they're smooth....but rough below that). But above idle the motor mounts wouldn't cause vibration to any notable extent.

I'd still lean toward balance shaft being off a tooth or more. Does it do it all the time, whether the car is moving or not?

It does it all the time, but it's hardly noticeable when the car is at idle. It increases with RPMs... even if the car is standing still.

[Davie Boy]

Originally Posted by Jfrahm

This sounds like a balance shaft issue and you should check the balance shaft phasing. If you cannot do this yourself you are going to go broke owning the car so it'd be good to start learning that sort of thing. It's a bad idea to run with the vibration as it can cause cracks in the oil pickup tube and that leads to oil starvation. The belt tension needs to be checked and re-set also after the belts run-in.

Re: engine mounts, How cheap is cheap? I put in the Meyle mounts and they are working great, very smooth idle. It appears that the URO mounts are Meyle with the name ground off, but that might just be the mold and not the rubber compound and filling. My concern about the Meyle mounts is that they may not last 20 years but function-wise they are fine.

Yeah, I went with the $45 mounts instead of the $250 OEMs. I expected a little more vibration maybe, but not what I'm experiencing. Thanks for the info. The mechanic seems to be in denial about the possibility of it being the balancer shaft - and he may be right. But as I said, he's not super familiar with these cars, so I'm taking that into consideration.

[Ga 951]

I expect the reason your vibration diminishes after warmup is that the Balance Belt streches a bit after it warms up.

My car has a BB whine when it is cold. After 10 minutes it disappears. I have tried so many times, but never seen to get the exact tension to stop it, either the whine, or too loose (up to the point of slapping the cover!). I have just learned to accept it...no big deal.

As you can tell, I still think you have a BB issue.
Of course, this advise is worth exactly what you paid for it

[Davie Boy]

Originally Posted by Ga 951

I expect the reason your vibration diminishes after warmup is that the Balance Belt streches a bit after it warms up.

My car has a BB whine when it is cold. After 10 minutes it disappears. I have tried so many times, but never seen to get the exact tension to stop it, either the whine, or too loose (up to the point of slapping the cover!). I have just learned to accept it...no big deal.

As you can tell, I still think you have a BB issue.
Of course, this advise is worth exactly what you paid for it

Thanks. I'm taking the car in this week to my usual Porsche guy to have him button things up. He thinks it's a balancer shaft issue too based on my description. I'll post a follow-up once it's complete.

[Davie Boy]

I have the verdict. It was an incorrectly installed balancer shaft belt. Thanks for all of your input.