996.1 transmission clunk - Dual Mass Flywheel issue?

Hi guys, first time poster after recent purchase of 996.1 with 85k miles. I have searched extensively on this issue so apologies if I have missed an existing thread. My issue is I have a delayed clunk on acceleration and deceleration. It feels like a delay in engagement of the drive to the wheels. I have replaced the engine mounts. Problem persists. I was told by my local specialist that it's the springs in the clutch, but from my research there are no springs in the clutch plate. He said I could drive safely until the clutch friction starts to go. My best conclusion is that the springs in the dual mass flywheel are worn. But I'm no expert. I have 2 questions. 1. Do you think it is the dual mass flywheel? 2. If yes is it safe to continue to drive like this? I don't have the funds to replace clutch & DMF right now, but I am also concerned that it is sending shock loads into the gearbox leading to much bigger issues. Any help or advice is very much appreciated as I am stressed about this issue. Thanks and hello from Ireland JT

The DMF does not use springs, it's two pieces bonded together with a rubber compound. It surely could cause clunking like you said you are having.

Hi JT. It would probably be helpful if you explained a little more about the symptoms (or lack of) other than the clunk. Is the clutch slipping (engine revs faster than the momentum of the car)? Do you feel a difference in momentum before and after the "clunk"? I am by know means an expert but my guess would be dmf or the clutch is starting to slip and the clunk is when the clutch plate fully engages (worn clutch plate).

Hi Guys, really appreciate the responses. ​​​​​​Interesting to know the DMF does not have springs, but that the rubber can wear out. I have no clutch slip at all. Just a delayed engagement on acceleration and lift off. If am smooth on clutch engagement I can hide the clunk, but it is hard to hide it on lift off. I can drive around the problem for the short term, just want to be sure it's not damaging the gearbox. Thanks! JT

Maybe clutch or differential? Cars are 20 years old. Getting my guard lsd rebuilt over the next week.

Maybe CV axles.... they do clunk.

Sorry guys, I've tried to respond but my responses are not appearing. Maybe it's because I'm a new member.

It's working now, fantastic. Hi Guys, really appreciate the responses. ​​​​​​Interesting to know the DMF has rubber instead of springs, but also that the rubber can wear out. I have no clutch slip at all. Pretty sure of that. She pulls very well through the rev range and can take off well when the clutch is engaged. I just a delayed engagement on acceleration and lift off. If am smooth on clutch engagement I can hide the clunk, but it is hard to hide it on lift off. I can drive around the problem for the short term, just want to be sure it's not damaging the gearbox. I do have the factory optional LSD. I'll look into how I can see if that is the issue. I'll also research how I test the CV joints. Thanks for the enthusiastic support! ​​​​​​ JT

Could be the pinion as well. I�d take it somewhere and have the mechanics start pushing and pulling on components and try to find the source of the noise.

Contact, any closer to finding out what it is? You got our attention.

Hi Guys, I've written 2 long replies and it says they need to go through the moderator. Bit frustrating.

Short messages are getting through so I'll break up my reply and post again in the morning. Appreciate everyone's the input.

So, definitely no clutch slip. Pulls and decelerates very well after clunk. There def feels like the drive is not engaged prior to the clunk. I can hide the clunk with careful clutch management. The problem is more obvious in gears 1,2 & 3, but I think this is mainly due to the mechanical advantage of the engine in those gears and the speeds I am travelling at (i.e. legal speeds

Very interesting to hear that the two parts of the DMF are connected by rubber, but that it could still be the problem if the rubber failed.

Thanks for the suggestions on the other potential sources, I will research how to test these. - LSD - I have the factory LSD - CVs - Pinion I'm booked into my Porsche Specialist in 2 weeks, so if I don't find it before then hopefully they will find it.

I'm very happy that I have people to engage with on this. Thanks everyone!

Transmission mount bushing.

What about engine mounts ? Or gearbox mounts moving under load?

I should have said it before. The first thing I did was replace the engine mounts as they were very worn. Other than improving shifting they unfortunately did not resolve the clunking. Will take a closer look at the gearbox mount.

The clutch in my 88 Carrera failed when the rubber came apart and wedged itself between the disc and flywheel, causing me to be unable to shift. It happened on the last lap of the last turn and last session of a DE track event. The car was nearly 20 years old then. But the rubber used on the 996 seems to be higher quality, and I�ve also not heard of many clutch issues with the 996 due to rubber, but I still wouldn�t rule it out completely. I�m with the person who said transmission mount as the next thing to check.

Quotes source post: 18104046

I could be mistaken, but didn't the 3.2 Carreras have a rubber centered clutch disk like the 944s, and not a rubber bonded DMF? The rubber centered clutch disks were a common failure point in the 911s and 944s that had them. The 996 has a spring clutch disk and has rubber in the DMF, which makes it a quite different failure and I think is in part why it is less common. To the op, I would recommend checking the gearbox mount as well.

All of the 996 clutch / DMF parts I have seen for sale have had a clutch plate with no springs. My interpretation was the "spring effect/damping" comes from the DMF. The only clutch plates with springs seemed to be paired with a single mass flywheel. As the single mass can't provide the "spring effect/damping" there needed to be springs in the clutch plate. This is only my interpretation based on reviewing the different options I could find for sale ​​​​​​

10 sec on your favorite Internet search engine yields relevant info to your Q 1: https://haynes.com/en-us/tips-tutori...hat-does-it-do When I replaced the clutch on my 99c2 at 115k miles I checked the DMF and it was within spec (max 15mm movement in each direction measured from rest and at the perimeter of the dmf) so I left it in. I put in a new transmission mount as well as both engine mounts. I still experienced the same behaviour as you describe (no clunk or noise but an oscillation that has two peaks before it's gone) including the bit that careful clutch in and out will avoid it altogether. I have put inserts into the voids of the trans mount but that did not reduce it at all. I have a hard time believing it's the dmf, I tend towards worn CV joints, even though they show only very little play.

Quotes source post: 18104536

You�re right, I misspoke. I was trying to refer to the DMF rubber.

Quotes source post: 18104787

Thanks for this insight. I'll keep you updated on what I find.

https://trade.mechanic.com.au/news/d...els-explained1

I'm not aware of any DMFs that use rubber to accomplish their function. I did a bit of googling and could not come up with anything about a rubber DMF either. Not to say they don't exist but it seems unlikely given the forces at work. That said, I would think that if the DMF is causing a clunk in the drivetrain, it's in really really bad shape.

An update from my Porsche specialist. He spent quite a bit of time looking into it and has concluded that it can only be coming from the clutch/flywheel. Now I just need to find a whole bunch of cash to replace those! I'll be sure to update this thread when the work is done, but it will be a while before I can afford it. Thanks

Ouch... Are you thinking "while you are in there", to replace a few more parts? Just been there myself.

Thanks for mentioning but I'm lucky that the previous owner did the IMS & RMS 6 months ago. He hadn't noticed the clunk, so they didn't check the flywheel when they were in there. The clutch looked ok, so they didn't replace that. At least I have a solid IMS & RMS Just found a very good deal on clutch & flywheel so making progress, and I guess the chances of any mechanical issues taking things apart will be greatly reduced as the box was off only 6 months ago!

Good news is you will be good for the next 20 years. Glad they figured it out.

Call me crazy, but I don�t know how anyone could pin the noise down to clutch/flywheel without physically looking at those items. Is it possible?

Hi All, apologies for the delay in updating, life has been very busy. I had the new flywheel and clutch fitted and there is definitely a big improvement. The play in the system is very much reduced. There is still a small amount of play, but I think this is the nature of the dual mass flywheel as the spec says it should have a small amount of movement from new. I do notice that the play is more pronounced further up the gears (3rd and above) but I have concluded that I am not accelerating as hard further up (speed limits) so the play is more obvious. Overall I am content and glad I did not replace the clutch and flywheel in error. I also had the gearbox oil replaced (MOTUL: GEAR 300 75W90) and fitted a gearbox mount bushing ( Powerflex Gearbox Front Mounting Bush Insert Kit PFR57-530) while I was in there. As a side note, the shifting is tighter and feels great. It was good anyway, but either the new oil or the new bushing (or both) are working well. No additional noise, vibration or harshness I can tell. Thanks to all for your input and support. Very much appreciated.

Thanks for the update! Did you experience a rattle with the old DMF when switching off the engine and revs dropping from idle to zero? That is the tell-tale symptom of a worn DMF as I was told by a local porsche specialist.

I did not notice the rattle or revs dropping. My guess is that the DMF was not totally gone, but was very worn. I need to check the play in the old one when I get a chance.

Sorry it took so long to close the loop. I tested the DMF and it moves quite easily by hand in each direction until it hits the stops ,(approx. 15mm each way). While it does spring back I'm pretty sure this must be the issue causing the clunk and play in the drive train. Thanks for all the input!