Goughary

So- driving along, no vibrations , or very limited, until approx 40mph and then severe vibration btw 40-45 mph. Then vibrations lessen as I travel above 45...

This happened with three different sets of wheels and tires. I thought all winter it was my snows, but I just changed to conti dw summer tires and it's as bad or worse on the summers. They are brand new. So still soft.

I thought maybe it was wheel bearings in the rear, so I took out my rear axles today. Wheel bearings are fine (new last summer). One axle has a tiny bit of play in the outer CV, but both were rebuilt by cv of Ocala last summer and they gave me a clean bill of health when they rebuilt them. I thought maybe a bent or out of round rotor, so I switched my rear rotors to my old set of sebro rotors that are like new. I had a function first transmission mount stiffener in, and took that out. Noise and vibration are still present.

Noise and vibration are the same at 45 mph with the brakes depressed. Noise and vibration are the same if the clutch is in and the car is idling at 45 mph. Noise and vibration are the same on varying road surfaces.

Tomorrow I'm taking the front axles out to check them. But I just can't get a handle on what this could be.

I had thought for a moment maybe it's the planetary gear, but that wouldn't be a frequency issue - i.e. My guess would be that if the planetary gear were going I would have vibration and noise through the entire speed range. And not just severe vibration at 45mph.

Anyone have any thoughts or similar experience?

Rocket Rob

My first thoughts were wheel balance or tires but you have eliminated that. Have you tried changing the side loads during the vibration by swerving left/right?

Steve Weiner-Rennsport Systems

Front? Rear?

LouZ

C2 or C4?

Look into the differential. Ever change the gear lube? What does the drive shaft look like ( one from trans to diffy).

I tore off all of the teeth due to low lube.

Goughary

Steve, I think it's the rear, but it's impossible to tell- the entire cabin vibrates.

Goughary

Gear oil is fresh delvac in the diff and the transaxle. If you run through suspension madness, when you get to my car, you see everything in the suspension and most moving parts are new. The axles were all rebuilt, but not new.

Could one outer cv in the rear with a tiny bit of play cause this much vibration? Cv of Ocala had given it the blessing...so I'm not sure.

Front end is getting taken apart today to explore.

Porschenut_Lee

I expected there to be a little bit more lag time between fix and break in the DBFD cycle.........

Yup, a CV axle with a little play can make pretty good vibration. Did you notice the vibration right after you got the car on the road last year?

Everything else on that car is new or refurbished. I'd start with the CV axle. Do you have a spare handy you can throw in for a test drive?

Porschenut_Lee

I didn't see anything above, but what happens if you put the drivetrain under load during severe vibration? Slight or heavy acceleration change anything?

C4inLA

This chap went thru hell eliminating sources, some scary that I would not do, but read his final post? He had warped rotors?

https://rennlist.com/forums/928-foru...-ideas-11.html

Goughary

I've got one spare rear axle- I'm going to send that off for a rebuild and install that in the place of the one I think may be problematic...if you drove past the area, I'll throw you the keys and you can experience it for yourself. I'm pretty sure it's the rear. If it were the front I'd feel it in the steering wheel. I'm pretty sure it's being transmitted to the body through the rear monoballs...so axles make sense. It's not the rear wheel bearings and it shouldn't be the transaxle or the diff cuz I'd feel that in the shift ****. Rotors are like new...and I swapped them for the set that was in the car. And with both sets I have vibration.

-nick

This is head-scratcher. Sounds like it began just before winter? Any projects at the time?

There aren't many things that rotate outboard of the trans/diff. You said that you suspect the rear, but I'll toss in some front contenders just in case. Fyi- My rear right outer cv has a slight play in it too. Just enough that I could feel when clamping down the axle and twisting the outer joint counter/clockwise by hand when the axle was out. But I don't have any vibrations at speed.

Have you replaced a seal on one of the trans/diff output flanges recently? Maybe you missed torquing a center bolt on one of the flanges afterwards?

Do you run wheel spacers? Maybe one of them is wonky?

Check that your inner tie rods are tight against the locknut? Check for play in the inner tie rod rubber fork connection?

The 993 folks get steering vibrations from worn front control arm bushings. I believe you're using fairly new Elephant RS bushings? Couldn't hurt to check the torque on the control arm bolts. And a visual of the bushings in case you the rubber-steel bond has separated on one.

That's all that I can think of Good luck!

Spyerx

GoPro mounted in a few different strategic spots to see if you can capture ?
Seems like an axle issue to me...

Goughary

Nick- i would love to have a talk and verbally knock back and forth the possibilities...it's causing me to scratch my head too...totally frustrating

All torques are good and right. But I do notice a little play in the diff axle flanges up front- though the bolts are torqued. I'm inclined however to think it's not the front- because I don't feel it in the steering wheel. So my head thinks the origin is the back.

Oddly enough - my car was like brand new before this became an issue. And now I want to drive it off a cliff. Terrible timing. I don't have the time at the moment to resole my own shoes....

Richard Curtis

by coincidence, i just read a Excellence piece on 964s and it mentioned "severe vibrations" occuring around 40-45 mph could be caused by bad front control arm bushings.

-nick

Another idea. For the heck of it, lift the e-brake a little and see if that has any effect. It would at least load up the rear wheels in isolation from the front.