Drivetrain vibration... What's next?
 

Hi guys,

Since I bought my car, I have a bad drivetrain vibration. I look through the forum for infos but now I'm out of idea.

The problem:
It happens when I leave a stop or a red light. The vibration is so bad that I'm almost not able to make the car moves. The whole car shakes instead of moving. Once it finally starts rolling, the vibration stops.

The problem doesn't happen all the time. It's random, but it seems to happen when there is a very light slope. Also it doesn't happen during the first 15-30 minutes of driving, so the car temperature seems to be related.

Facs about my car:
- 77000 miles on the car.
- New engine mounts.
- Used clutch that looks fine.
- PPF not cracked.

This week I dropped the diff to verify the diff mounts. I attached some pics of the mounts. There are very small cracks that are no more than 1mm or 2mm deep. I really doubt the vibration is due to these tiny cracks. To me they are in pretty decent condition for a 20 years old car. As we speak now the diff mounts have been replaced but I can't try the car since there is snow here.

Last winter I rebuilt the engine and re-used the clutch/pressure plate combo. It looked fine but I could be wrong. Right now I'm thinking the clutch may be the culprit.

But before proceeding I'd like to have your inputs to know if there is something else that could make these vibrations.

Also do you guys think these small cracks in the diff mounts could be the cause?

Thanks guys and happy new year.

If memory serves, the diff mounts are liquid-filled. If they're bad, they'll be leaking.

Really sounds like a chattering clutch or something in the rear suspension that's getting into a harmonic. How do the other rear bushings look?

Dale

I didn't see any traces of liquid around the diff. This makes me think the bushing were fine.

Honestly I'm not too good when it comes time to verify suspension bushings. I did a quick check with a pry bar and everything seems stiff except the I-arm that I was able to rotate from one side to the other with my hands. Is this normal?

Had the same problem and it was my PPF. It wasn't cracked but it was twisted and this caused the same exact symptoms you describe. Had to look close to see the problem. Replaced it and problem solved.

I think that you're talking about the trailing arms? While I doubt that these are causing your problem, the bushings in the arms are inexpensive to replace.

Energy Suspension 11.7103 Rear Trailing Arm Bushings

(Credit to Dale for finding this.)

Subscribed. I'd like to know what's causing this.

Very interesting. I'll check again closely for anomalies. I guess it's hard to see when you don't have a correct one beside to compare.

If I refer to the FSM at page R-36 it's the part called I-arm. It's also called the rear lower arm which I think refers to the whole assembly. The trailing arm is connected to the I-arm. Anyway I'm talking about the one with the two pillow balls. I can rotate it with my hands. Normal?

Thanks for the link.

Take a look at this post:
https://www.rx7club.com/3rd-generati...8/#post5604821

I'd call it normal. The I-arm has two small and a large pillow ball bushing. I've never tried to move the large one but the smaller ones move freely. I would assume a larger one does. Pillow Ball bushings are basically a ball bearing with a hole drilled through the middle.

if bushings are the issue then you would have vibration issues all the time, not just temp related. i doubt your issue is in the suspension. i doubt its something out of balance too. i also doubt its your clutch. ive never heard of a clutch producing a vibration so bad the car cant move, they either work or they dont. now if its dual friction clutch maybe but we aint got that shit on these cars. Im leaning toward engine managment issues.

Yep, I would never have known it was bad. It bent slightly over time and caused the very same issue you describe - Car would idle just fine but when I tried to engage the clutch to take off, extreme shuttering. Lasted until I went into second gear and then all was good again. Thought I had blown something.

I just put in all new bushings in the rear end (thanks Ray for the smoking deal). These cars are now twenty years old and it made a huge difference for me. As for the diff bushings leaking you may never know they leaked. I just happened to get lucky when one of mine went. It was a small amount of clear fluid that I happened to spot one day. As mentioned earlier about harmonic vibration it is a hard one to track down. Once the conditions are in place it sets that off and it just gets worse. It seems your heading in the right direction with the drivetrain and clutch, especially since it is bad under starting out on a slope.

Did it happen everytime or randomly like me?

Little update.

Today I drilled a small hole in the old diff mounts I removed last week to check if there was liquid in it. Old mounts were definitely good so the vibration is not coming from these.

So I think two options left:

- Twisted PPF.
- Clutch.

I took another look to the PPF and I see no anomalies so I'll have to buy a correct one to make sure mine is twisted or not. I would not be surprise if it's twisted cause the engine mount on the driver side was broken for a few years. For sure I'd prefer this option than replacing the clutch :P.

And the winner is David Hayes for the twisted PPF (at least I'm 99.9% sure).

Today I compared my PPF to a good used one. When I drop off my PPF on the concrete I noticed that it just didn't sound right. The good one sounds a lot more solid and a lot more in "one piece".

If you look at the pics, I took a measurement on the diff side of the new PPF. The good one measures 2 inches and mine was at 2¼ inches.

It was impossible to notice the distortion without comparing to a correct one.

I guess that save me a clutch job...

Jus sounded too much like what I went through. Hope that is it as it is an easy fix.

Tonight I drove the FD for the first time of the summer and the vibration is still there.

To recap:

- new motor mounts
- new diff mounts
- replaced the PPF
- clutch doesn't slip

Do you guys have any input?

I'm starting to think the diff could be the problem since the vibration seems to come from the rear-end.

Does a bad diff could cause that kind of vibration?

Thanks

Have you checked the u joints in the driveshaft?

Could be a warped or cracked flywheel?

My guess is the driveshaft and the u-joints. I believe you can bring the driveshaft to a shop to test for balance also.

I put the car on stands. I tried to shake the driveshaft and it doesn't move very much. However I have not removed it to inspect it.

I don't think so. Last winter I rebuilt the engine and the flywheel was fine. I also resurfaced it during the rebuilt. The vibration problem was there before I rebuilt the engine.

A little update.

I put the car on stands. I inspected the driveshaft and the u-joints. All seems fine. I drained the diff to check for metal flakes. No flakes at all. The oil looks almost new. So it's probably not the diff.

I went under the car while a friend of mine put the car in reverse and engaged/disengaged the clutch while the car was running). While engaging the clutch in a fast movement the whole drivetrain shook like crazy. I look through the inspection hole under the transmission and the vibration seems to come at a specific clutch/pressure plate position.

So IMO the suspect is the clutch/PP combo.

What do you guys think? Someone ever had this problem before?

Testing the car unloaded you will often see a lot of vibration, especially if you release the clutch in one fast movement as that sends a shock through the drivetrain.

If it is clutch shudder you normally only feel it as the clutch is engaging, once fully engaged all shudder would stop. Clutch shudder can be caused from over heating or contamination. Some high performance clutches it is almost unavoidable.

I had a similar issue with a Torsen diff on a Miata, there would occasionally be a light shudder on take off and no other time. This shudder was felt as the car transitioned from stopped to moving and stopped shuddering very shortly after the car was rolling. Upon inspection with a borescope through the fill hole I saw a the surface of the helical gears in the Torsen were extremely scarred or chipped in a few places. Eventually the shuddering got severe, almost to where it was hard to even start moving, like it was locking up. Eventually the Torsen shot a chunk through the rear cover. From my experience with Torsen diffs, they mainly fail from abuse, like too much wheel hop, 300ft/lbs torque and lots of drag launches. Even at high torque levels they seem to hold up great as long as you avoid wheel hop. Basically avoid shocking them.

I have seen similar issues with u-joints locking up or failing. Often this starts on initial takeoff. As the joints get worse the vibration/shudder gets worse and more frequent under various conditions.

To check I would see if the vibration was only during the engagement process of the clutch. That would most likely mean clutch shudder. You should be able to look through the fill hole to inspect the diff gears for chipping. Next remove the driveshaft and check for binding in the joints or joints that are too loose feeling. With age of the car it may not be a bad idea to have them replaced and the shaft rebalanced depending on the cost. I've had driveshaft shops do this for as little as $100-150.

Thanks jfantis for this detailed answer.

This is exactly what is happening in my case. When loaded and unloaded.

This is not my case. The car did see road course but not drag racing and I'm not at 300 ft/lbs. The only wheel hop/vibrations the diff sees is the current problem.

If the gears were chipped wouldn't I've seen metal flakes on the drain plug when I drained the diff last week?

I still think it's something with the clutch. Pull the trans and see what you have!

Dale

Well sorry it wasn't the PPF. Wounded exatly like my issues. I know go with what Dale says. Never bet against th big man :)