jcan

I've got a 1999 C2 Manual 996 with a mystery "thud" from the back. It only happens on acceleration from a stop to first gear, not reverse, or at least not that I can tell.

If I drive nicely away it can do it, as well as on hard acceleration. It doesn't happen instantly, maybe a half tire revolution or a couple feet forward then thud... It only sometimes happens though.

I've disconnected the rear sway bar links and checked them for play as the front ones needed replacing but the rears seem fine. I've read a lot of the old posts and it seems to point towards the engine mounts. Only thing is that they "look" fine, no oil or weeping.

I don't think they've ever been changed though in the 80,000 miles of it's life. If I was to pull one out would it be evident then that it was worn or in need of replacement? Can these mounts be no good and yet look fine or could this be a bigger problem?

Thanks in Advance,
Jeff

moonshadow

I had the same problem with my 2003 996 cabrio and it was the transmission mount, it is actually a rubber insert,cost me about 600 to fix

jcan

I'm noticing first thing in the mornings it's fine ( car cold) and then after about 10 minutes of stop go traffic it starts to appear. I wouldn't say I roll a couple feet forward but there's a definite delay to when it thuds. When you say transmission mount is this the one that needs to be pressed out?

jcan

To be clear this is on my c2 911 manual car

It's been awhile trying different ideas to see what is causing this thud. I've replaced both Engine mounts to no avail

I've filled the transmission mount with silicon thinking if it were the problem it would make it better and for awhile I thought it had... The thud was quieter and not as metallic sounding but still there. What I did was take most of the silicon back out in thinking the sound should be back or at least worse... Nope.

It's still the same clunk/ thud, but no where near as consistently bad as it was a few months ago.

The only thing that is changing that I can figure is that the cars suspension is being extended every time I jack the car up. Would this maybe cause something to shift or sit differently each time??

What I have noticed is the left rear shock does have oil that has soaked into the dirt of the shock absorber, looks wet. The right one is "dry" looking. would a bad shock give this noise?

Apart from this my car is running great!! I don't chime in too much at all but I do follow all the posts so thanks for all the laughs

MetBlk 911

If it were me, and I was trying to save some money, I'd take it to an independent Porsche mechanic for diagnosis and then do the work myself once the problem was determined.

Pac996

Let us know what comes of it. I get the same thing when getting on it. Shop says motor mounts are ok and nothing else appears wrong. Maybe just the mounts are sounding off when they hit the max compression. The stock mounts are kind of soft from what I've heard so slamming them to max seems possible.

farfinator

Basalt 911-For real? Please, some self respect. Is this a Porsche or Geo forum?

MetBlk 911

Sorry farf, you lost me. What's your solution to the problem?

farfinator

Well, the first thing I would avoid is advocating poking the hardworking independent in the eye and suggesting anyone else do same. We are all big boys and if an issue arises that is beyond our skill, that's the time to suck it up and PAY the professional.
But, if I were to suggest an initial course that may afford some additional incite:

1. Have you retorqued all the suspension and chassis bolts?
2. Before you siliconed the transmission mount did you observe any compression/tearing to the rubber joint?
3. Had you had any service prior to the the appearance of the thump that may be suggestive?
4. While coming from the rear, do you have any sense as to whether it appears to be more pronounced left, right or central.
5. Could you take a friend with you and sit in the back to try and refine the vicinity where it is most pronounced?
6. Yes, most assuredly, if there is a loss of oil in a shock it very well could, particularly after modest warm up produce a thud, but I would expect it to be justified to that side of the car.
7. Does it transmit through the shifter or through the chassis to your butt?
8. Is it audible or just a feeling?

But before, I went through all that I would certainly recognize that 80K leaking shocks warrant replacement and would resolve first to replace and then re-evaluate and eliminate a known failed part as a contributing factor. And, before I resolved to do this myself, I would hope to be confident in my ability to recognize a bad upper strut mount, before missing an opportunity to fully resolve the issue.

If shopping for suspension:
The PPS10 kit special at Tire Rack is a pretty good deal at $2495.

Alternatively if budget is critical:
Rear Shocks 996-333-051-04 Bilstein HD can be gotten for $200 ea
Rear Upper Mounts 996-333-059-04 for about $150 ea.

jcan

I'll go with the shocks first and see how that all ends up. I they need replacing as the front were done not too long ago.. I'll see if that fixes the clunk. Thanks again

MetBlk 911

I see where you are coming from, but I wasn't suggesting that he try to get something for nothing. I assumed that he would still have to pay the indie for diagnostic time. Frankly, my first response was going to be just what you said. If you can't figure it out, take it to a professional, but it was obvious that he wanted to go the DIY route, hence my recommendation to go half way and pay for diagnostic, then do the work himself.

spender

Hi jcan - any updates on this? I've got a 2003 C4s, with similar symptoms to what you describe (clunk when driving off in first gear). My mechanic says that it isn't the mounts and believes it's related to the brakes.

Some things that threads on renntech and rennlist have suggested include:

- check CV joints
- cracked wheel bearing
- sway bar links
- tranny
- shocks (need to be tightened)

Good luck, and let me know how yours shakes out.

jcan

Well I've changed the shocks and drop links over and it rides nicer but the clunk/thud is still there. I may get under the car today and have another look... The main problem is that it's very intermittent. I have tightened some of the transmission mounts but I'll just go over them all to be sure to be sure. I know one of the bolts holding the transmission was loose and that fixed the more metallic clunky sound, now its a softer thud. Engine mounts have been replaced about 10k ago so that should be out as well. It's bearable for the time being and I'll find out soon enough when it breaks or completely wears out and I'm up for big money

jasper

You know Jeff....I'm starting to think this might be your dual mass flywheel.

My Audi Allroad sometimes has a bit of a clunk, although I feel it more than hear it. Looking back through the paperwork I saw that the PO complained to Audi while the car was under warranty and that was the diagnosis. They said it was normal but it isn't really...not if the 996 is any indication.

I still haven't taken a ride in your car after all this time. Maybe we should actually do that some time eh?

redridge

check you brake lights.... yes they loosen sometimes and can cause a clunk. Put some shimmy between the light and frame and make sure it is nice and snug. GL with the search.