Slow Speed Clunk from Rear

[Maffett]

Well after owning a 74 911 for 18 years I find another Porsche in the family. I bought an 86 928 from a friend last week. It's got 150k miles. I am still very much getting used to the car, but it seems to be a solid and reliable vehicle. The car came with good maintenance records. The timing belt has about 10k on it. The water pump about the same. The distributor, plugs, and wires are fresh as is the master cylinder and brakes. Wheels and tires are in good shape. The car starts easily and drives great. Although it has about twice the HP as the 911, I can really feel the extra weight.

The only concern I have thus far is a clunking noise coming from the rear as I hit small bumps at slow speeds (the slower the louder). Higher speeds do not cause the noise, or it is overshadowed by the general louder background noise.

I pulled all the stuff out of the trunk - tools, jack, tire, etc. I strapped down a loose battery. No change.

I jacked one of the rear wheels off the ground and manhandled the wheel expecting a loose bearing. No noticeable wheel movement.

Where else should I look? I can't find in the maintenance records if the rear shocks have ever been replaced. They don't function as if they are shot. But could they be making the clunk noise.

Thanks in advance.

[928 Bro]

Are your trans axle boots ok the joints might be going bad and you hear it because the bumper, or speed change if there is play in the joint normally at first it will be slight and increase the worse the play gets. Just an idea

[Barbapapa]

The trans mounts could be it. Motor mounts too as it's all one piece. If your trans mounts are shot you could hear the trans contacting the crossmemebr at the bottom.

[Maffett]

Thanks for the replies. I'll look into the boots and mounts. I have yet to pick up the shop manuals from the previous owner, so I don't yet dare to get the car up on jack stands. I wouldn't know what to be looking for. (I'm used to the engine being back there)

As I think about the noise a little more, something like the transmission mounts feels more likely to be the problem rather than the shocks. If it were the shocks, a higher speed bump should just make a louder clunk. The shocks would have to work harder at higher speed.

But if the transmission mounts have come apart, ( I found a photo on Pelican's sales side and the mounts appear to be a rubber / steel composite ), a higher speed bump may not jar the transmission vertically as much as a slow speed bump.

If I find out anything interesting I post it.

[Maffett]

Well a little progress. I managed to get the rear end off the ground and figured out where to put jack stands. I read somehere on this board that it's OK to push up on the cross bar beneath the transmission. With a wood shim on the floor jack that idea worked pretty well. (Is there an similar spot in the front where a floor jack can push up? I don't see how to raise the front of the car by using the front rocker jack points and then place a floor stand because the jack is in the way??)

I found the transmission mounts and they seem to be OK. The rubber seems to be uniform and connected to both upper and lower sections of the mount. The transmission does not appear to be resting or hitting the top of the cross bar. I used the floor jack to push up slightly on the transmission and watched as the mounts stretched a bit (1/16"). The mounts look fine.

I shook the exhaust system and was amazed at how much movement it could take. But no amount of shaking made it hit anywhere. The rubber hangers squeeked a lot but I lubed them a bit and the squeeks went away. There is a broken rubber hanger in the middle of the car where the two exhaust pipes are rounted close together. I'll get that replaced, but I don't think it factors into my clunk noise.

The axles and boots look great. They appear relatively new. So that doesn't seem to be the source.

I put the car back on the ground without knowing much more about the clunk. I then stood on one of the rear bumperlets and jumped up and down. I can move the car about 1" (I weigh about 160 lbs). When I do this on the driver's side, no noise. On the passenger's side I hear a catching and relase kind of a noise. It's almost like something gets stuck and the snaps to release. Both moving down as well as up. It's not a hitting sound. I'm back to wondering about the shocks, specifically the passenger side.

Would a shock fail so that it would catch and snap before moving?

[Maffett]

Well I'm getting closer to the source of the noise. I'm focused on the passenger side / right rear shock.

I had my daughter jump up and down in the rear of the car while I crawled around underneath (the car was on the ground at the time).

The sound is made each time the car movement changes direction - up / down and down / up. The only source appears to be the shock. The transmission mounts, exhaust system, and other components appeared to be OK.

I took the shock off. It's a Bilstein F4-B46-0974-HO. It appears to be original equipment. In hand, it's difficult to tell if anything is wrong with the shock. It does have a plastic dampener that floats up and down on the shaft that makes a little noise when I shake the unit up and down, but it's hard to think this is the problem. I will need to procure a set of spring compressors next.

I'm guessing that the piston has come loose inside the shock cylinder.

Can I rebuild the shock myself, or is this something to send off and have done elsewhere? I guess I should do both rears regardless.

[MPDano]

I would get them them rebuilt. Actually Bilstein is here in San Diego, Calif (Poway). Good job in utilizing a family member in your troubleshooting. Someone on here can give you a roundabout price of how much they cost to rebuild by Bilstein.

[usfromflorida]

Did you check your parking brake and see if it is stuck or loose?

[Maffett]

Parking Brake? I'm not sure if I understand the connection.

I've only tried the brake once since getting the car, and it only partially held. I do know that the rear wheels rotate free enough when they are off the ground (in opposite directions thru the differential). I have put the car in neutral and pushed it around in the garage also. It's far more difficult to push than my 911. But I guess that's to be expected.

How could the parking brake cause the noise?

[Danglerb]

Avoid a cheap shock spring tool, find a local parts store that loans a good tool, or just have a shop do it. I doubt anything in the shock is making the sound, but Bilstein rebuilds shocks for $65 plus shipping each way. All the info is on their web site. I'd send them in pairs as well. $25 for just testing them IIRC.

Anybody local you can borrow a good used shock from, ideally with the spring still on it, and check you car with it?

[928gene928]

Maffett,
I had a similar clunk at the rear several years ago, and it was one of the shocks. It took some time to chase it down. I made all the check you mentioned, and finally found the problem.

[Danglerb]

The shock was clunking internally?

[Maffett]

From the Bilstein diagrams I've seem, the piston is bolted to the rod. I'm guessing that the nut has backed off a bit and allows the piston to float on the rod. If this is true, will I be able to tell once I have the spring off the shock? I would think that I should be able to push / pull the rod and feel the movement.

[Maffett]

Danglerb, from the Daughter jump test, the clunk appeared to be only coming from the shock. Both top and bottom shock connections are fine.

It's hard for me to be sure the that the sounds I hear during the Daughter jump test are the same as I hear / feel on the road. But it stands to reason that they are related.

[Danglerb]

I've been fiddling with some shocks lately, bought some used, put one on my car, and generally attempted to check all of them. With the spring off you can compress the shock against the garage floor by leaning on it hard (I'm a big guy, so easier for some than others), and all of mine behave about the same slowly compressing, and slowly expanding when released.

You "might" be able to fully compress the shock and then somehow pull on the rod to check for movement between it and the piston?

I've actually been thinking about building some kind of cheesy shock tester, but no action yet, still looking for a cheap local test place. I started a thread about a month ago on shock dyno testing.

[Maffett]

Well, the noise, or should I say, a noise is coming from both rear shocks. I took them off over the weekend, and against better advice, bought a cheap pair of sprinng compressors to get the coils off. The coils came off without a problem. Makes me wonder if the springs are shot. They sure don't seem to be and the car rides OK.

Anyway, the problem is that the upper shock mounts have been loose in relation to the shock rod. The mounts have been able to hammer up and down on the rod. Over time this has damaged the shoulder of the rod and the washer that sits on the shoulder.

I put everthing back together and tightened the lock nut on the end of the rod as much as I dared. This reduced the noise considerably, but I will have to replace some parts in order to eliminate it.

When I get the shocks refurbished, will they replace the shaft? If not I will need to buy new shocks. I will also need the washer that sits on the shock shaft.

I'm also woindering if the upper shock mounts, the ones with the large conical shaped rubber bushing is bad. I can easily remove the bushing from the conical metal holder. I get the impression that orginally the two were fused together somehow. If they are now separated, is this part of the problem?

[MPDano]

Great find!