Mark Hubley

The Car
1987 924S just shy of 58,000 miles. I've owned it since March 2011.

Symptoms
Significant vibration between 3,200 and 3,500 RPM. I can feel it in my butt and hear it. Most noticeable is vibration in the gear shifter. Were I to put my hand on the shifter at this RPM it wouldn't be long before my hand went numb. I notice nothing odd at idle, and there is a bit of vibration as I go up to 3,200, but I probably wouldn't have noticed if I wasn't looking for it. Vibration quickly gets serious at 3,200 and drops off dramatically over 3,500.

Vibration is present while driving and while revving the motor with the car sitting still in neutral. If I get the motor to 3,200 RPM with the car sitting still and disengage the clutch, the vibration continues.

While driving, if I quickly lift off the throttle, the car gets just a bit of a jolt and I can hear what sounds like a bit of a fart sound from (I believe) the exhaust.

Some History I Think is Relevant
When I bought the car it came with an aftermarket exhaust. Records place purchase in 1998. The muffler looked pretty rough when I bought the car. July 22 of this year, turn 5 at Summit Point, I suddenly hear an awful racket coming from the back, and lots of vibrations. I'm thinking the muffler finally bought it, or I have a transmission problem. After I get off the track I take a look in the tailpipe, and I can see debris at the outlet of the muffler. Better a dead muffler than a dead tranny. I drove the car home 100 miles from the track, it sounded awful across the RPM range, and the power was down, presumably from the back pressure of the obstructed muffler.

Yesterday I took off the old muffler and replaced it with a new Bosal stock muffler. This has eliminated nearly all of the bad symptoms I was experiencing since things went bad on the track. However, I'm left with what I described above. So, the new muffler didn't solve everything. After removing the old muffler I did an autopsy, cutting an opening into the muffler. Here's a pic:



There is a pipe running vertically down the middle of the muffler, and you can see that a large portion has broken loose. This was at an angle (more pronounced than seen in the pic) that was obstructing flow of exhaust out of the tailpipe. In this pic, the tailpipe would be extending downward.

Speculation
So, there was definitely a problem with my muffler, and replacing it got the car almost back to normal. But what's causing this vibration that I'm left with? When I first had this problem at the track, someone suggested it might be my catalytic converter, rather than the muffler. The main reason I did my muffler autopsy was to see if I could find any evidence of debris from the cat inside the muffler. I didn't notice anything of that sort. That being said, I still have my suspicions about the cat.

It seems unlikely to me that two completely unrelated problems would happen at the same time (thought it's possible). Perhaps there is some relationship between the muffler going bad and something happening with the cat? Maybe driving with an obstructed muffler for 100 miles damaged the cat???

If something is off with the cat, I imagine vibrations from the cat could easily be transmitted to the torque tube and the shifter.

I think the fact that the vibration occurs with the tranny disengaged from the motor rules out a problem with the transmission. Maybe not? And I just don't think it's a problem with the motor itself, but maybe that's just a gut feeling.

Is there any way to test the cat? I guess I can drop $120 on an aftermarket Catco cat and hope that solves the problem.

Any advice is welcome

Fara

I had a very similar problem with mine

It turned out to be a missing exhaust stud (nearest the firewall)
The car was purchased with the problem - just took a long time to find it

Edit: Just suggesting you check your manifold - it may be under torqued on one or more studs.

Mark Hubley

That will be easy enough to check out. That makes me think it could be something as simple as me needing to retighten the bolts where I attached the new muffler, now that I've run the car.

Fingers crossed that it is something this simple!

JohnKoaWood

3200 is indicative of a balance shaft out if time... Set the engine to tdc at the cam timing mark, and verify the upper and lower balance shaft timing.

Next would be to check motor mounts, but they are less likely!

John_AZ

Did the PO install or replace the clutch before you bought the car?

I have an '87 924S.
Before I replaced the rubber clutch disc I had the "normal" vibration at 3000-3200rpm with the clutch engaged.
The vibration is caused by the missing rubber on the clutch disc.

If the PO replaced the rubber clutch disc, he may not have had the FW flywheel ground or balanced.
This could be causing your vibration problem now.

John

Mark Hubley

I've wondered a bit about the clutch. There is nothing in any records about the clutch ever being replaced. From what I can see through the bell housing, the disc is plenty thick. However, I know from experience with the 944 I used to own that the rubber center can disintegrate. I can imagine the stress of driving on the track or driving with an obstructed exhaust possibly pushing the clutch over the edge. However, do you think if the clutch were the problem that the vibration would be unchanged when I disengage the clutch?

Considering that the car had been driving without any such vibration prior to a couple of weeks ago, I not inclined to suspect the flywheel.

Having done the clutch replacement myself on that 944, I'm going to put myself into denial (as long as possible) that this could be the signal that it's time to replace the clutch in this car!

Mark Hubley

I can check the balance shafts easily enough. I just replaced the front seals a couple of weeks before going to the track, thus I removed and replaced the timing belts. The car made the trip to the track and two days of track sessions with no unusual vibrations. So, unless a balance shaft skipped its place on the belt, I think that should be OK. But I can check them.

I put new motor mounts into this car shortly after buying it last year, and I've been through motor mounts in the old 944. The symptoms I have now are very different from the motor mount vibrations I've experienced before.

Thanks guys for the replies!

As soon as I can (maybe tonight) I'll get the car off the ground and have my wife step on the gas while I take a look underneath. Maybe I'll be able to see something that looks out of whack.

ECM MAN

Having the same issue on the same model and year. Vibration not bad, but very annoying and getting worse. Please let us know what you find! I'd rather not reinvent the wheel.

Thanks!

bonus12

My 951's exhaust was rattling against a heat shield. It was RPM-specific, at a similar RPM to yours. I could feel it in my seat, and it was like a thumping coming from under the seats, until I replaced a missing hanger. I recall being surprised that a rattling exhaust could make such a noise! But the rubber hanger secured the exhaust pipe and solved the issue.

If this sounds like yours, fit it fast; it'll only get worse!
I was lucky because it was the first thing I checked. A shop lifted the car for I think $20.

Mark Hubley

I have a diagnosis; won't know if it's correct until I attempt a fix. Should have had this in mind before my first post . . .

I purchased a new muffler and a new ring seal that goes where the pipe from the cat bolts to the pipe from the muffler. When I removed the old muffler, I noticed the ring seal was seriously corroded to the pipe from the cat. Having no torch or welding/cutting skills, I left in the old seal and just bolted up the new muffler. It seemed tight, and while the car was still in the air I started it up and took a look. Things seemed fine, but that was just at idle.

Today after work I put the car back on the jack stands, fired it up, and had my son work the throttle while I looked underneath. As the revs went up, water spurted out of the junction between the cat and the muffler, so that seal clearly is not tight. At ~3,200R RPM the exhaust system back of the junction started vibrating quite violently, and the vibrations went away at higher RPM.

So, I'm thinking my problem is as simple as me not getting that junction tight. I'm thinking I'll take the car to a shop to see if they can get things cleaned up, and maybe get the new seal in.

I'll update later.

Calmchaos

Did you check to the balance shafts? The engine could be shaking the exhaust really hard.

Dougs951

Let us know if that solves it!

Oddjob

Typically, exhaust doesn't shake the engine, the engine will shake the exhaust. Check for a broken balance shaft belt, or possibly a bad motor mount.

MAGK944

Have to agree that this seems like a balance shaft issue. Typically a balance shaft out causes low frequency vibration from idle up to 3500, just like you are feeling. It's barely noticeable at idle, worst at 3200-3500 and goes away above 3500+

As you said you changed the belts and had no issue for a period of time, your belts must have been aligned properly but the balance belt has skipped.

Exhausts cause rattles, balance shafts cause vibration. If you can feel it through the seat you have vibration.

Unlikely it's motor mounts they tend to be felt more at idle/low rpm.

Campbelljj

I would also suspect the clutch. My 89 recently starting vibrating around the 3,000 - 3,500 RPM range. Since I knew all was fine mechanically with the engine (incl. bal shft timing, belts, exhaust) I suspected the clutch was the issue. This proved to be correct as the spring style spacer (the ring that compresses with the castellated fingers) for the TOB had actually sheared in half allowing the TOB to vibrate. Not saying this is exactly what happenned with your car, but it did with mine. I have since replaced the clutch plate, pressure plate and TOB with a new SPEC setup and the vibrations are gone. Lot's of luck!