Stange thump while 911 flexes....

This is driving me nuts. I have been looking for this noise for over a year now.

I get this random "thump" noise, kinda like something is loose and hitting a surface when my 911 is flexed, ie over speedbumps, bumpy roads, etc.

I have had the entire rear and front suspension out and everything looks fine. I took it out to put in larger t-bars, adjustable spring plates, 22mm welt sways, new shocks. Check the sway mounts, banana bushings, bearings, etc..

I can only think of three things left.

1 - Motor Mounts. The car has 125,000 Miles.
2 - Upper shock mounts
3 - Slop in the solid bushings in the adjustable spring plates (sway-aways).

I am now looking at the motor mounts.

Question: How to I tell that the motor mounts are worn? I am going to take them off today to see, but I don't know what amount of slop is expected? Any?

Any other ideas?

Please save me from insanity!

Cheers,

Mike
83 911SC

Motor mounts were my first thought. I would just replace them with the Club Sports mounts simply due to age alone. They are better than the originals as a side benefit. This will increase the chassis's torsional rigidity for improved handling and make shifting more predictable, especially under hard driving conditions.

Other thoughts are steering rack and steering shaft parts. I had a strange clunk in my 72 over sharp bumps that caused me to replace everything. I finally put in a new rack. What I found when I went to put the new rack in was that the flange that splines onto the pinion of the rack had a loose attachment nut. this made the splines loosen up causing a clunk in my steering. The rack was actually fine but I replaced it anyway. (Wasn't too confident in the spline condition)

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Tyson Schmidt
72 911 Cabriolet
92 C-2 Cabriolet

Check C/v joints if coming from the rear

Is it from the front? Aftermarket sway bars with some play side to side will knock when the bar slides and the end stops make contact. This is usually always from the front.

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8 9 9 1 1, The last of the line.

Well, I think its from the back, or at least it sounds that way. Sometimes the sounds are misleading. The front feels pretty solid, so I don't think its the steering rack (although I will check).

I just put in new rear bearings, and while I was in, the CVs were pretty worn, so I put in new CVs. I don't think its the driveline.

I am homing in on the motor mounts.

Does anyone know what is the difference between the Pelican "sport" mounts and the stock ones (besides the sport ones are cheaper)? I doubt if they will roughen the ride much since I am already running sport shocks, 21/28 mm tbars and 22mm adjustable sways, which is pretty firm but tolerable on rough roads. **but does it corner!**

Oh, before I put in new bearings, front shocks, 22mm sways, CVs it used to make the same noise, just not so often. Now it does it all the time which means when I do fix it, I can tell! ;-)

BTW, really thanks for the help!

Cheers,

Mike
83 911SC

some guys jack the engine up with a block of wood under the oil cover. I would think you could get under there while having someone jacking, and see if the mounts lift before the car does....this should answer your ? about the mounts being a problem.

Luck

Don't forget to check the trans. cross member mounts too. They are the same part as the motor mounts.

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Tyson Schmidt
72 911 Cabriolet
92 C-2 Cabriolet

I had the same kind of situation with my '72. It turned out to a loose nut on the top of the passenger side shock.

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Marv Evans
'72 911E

By "see if the mount lifts" I think you mean is that normally the tranny is hanging by the mounts. If I have the car up on car ramps, and put a jack under the tail of the tranny, the mounts give a good 3/8" before the top of the mount comes in contact with the body mount and the car starts to lift. Essentially, the tranny is "hanging" off the bolts.

Question: Is the rubber of the mount in contact with the body normally? I have nothing to compare to.

Thanks!

Mike

Originally posted by Tlook:
some guys jack the engine up with a block of wood under the oil cover. I would think you could get under there while having someone jacking, and see if the mounts lift before the car does....this should answer your ? about the mounts being a problem.

Luck

If you are getting that much movement before the car lifts, then you need new mounts. Whether or not that is your clunk is not certain, but you do need new mounts.

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Tyson Schmidt
72 911 Cabriolet
92 C-2 Cabriolet

Is any movement okay, or should the mounts be tight up?

Cheers,

Mike

Very little movement is OK. Are your big washers installed concave or convex? If you install the washers with them cupping the mount, it will tighten things up. This was a factory recommended fix.

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Tyson Schmidt
72 911 Cabriolet
92 C-2 Cabriolet

Hi Tyson,

The washers are concave down, that is the outside edge of the washers touch the mount first. This means that they are already in the position that pushes them the most.

The distance they move is not slight, its quite a gap. Did not measure, but its around 1/4 inch or so. Less than what I stated before, but enough so then the engine is unsupported in the car, you can see a gap between the top of the rubber mount and the mount on the body of the car.

Will look at ordering new mounts today.

Cheers,

Mike

Woops, I mean concave up! ;-)

I've been through the transmission mount problem but it won't give a "random" thump.

To check it out get the car into second gear and accelerate quickly, this should drive the transmission mounts up to the body work. Now take your foot off the gas quickly which will let the transmission mounts fall. Do it a few times jerking the car between acceleration and deceleration. If the thumps are in sync with your movements change the mounts on the transmission bar and make sure the tranny is firmly bolted to the bar.

Mounts are about $40 each from Pelican and it's an easy fix. Jack up the rear of the car and put it on axle stands. Support the transmission (I use wooden blocks). Remove the mount bolts and change the mounts.

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Allan Broadribb
'70-911E, 2.2l with Webers

Same thing happened to me... come to find out it was my personalized "Al Pachino" Bowling ball I had left in the trunk after a night of drinking Irish Squids and Pocahontas Phili Puckers...

Ekk-Gads! "did I say that out loud?"....

often the rear swaybar is too close to the trans mount and they knock over bumps. a length of rubber hose, split lengthwise, can be attached to the bar with zip ties.

My sense of humor is way the f#@$# out there, but Leland scares even *me*. I'm starting to feel almost normal.

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Tyson Schmidt
72 911 Cabriolet
92 C-2 Cabriolet

I will try this. It may be hitting the bar given the mounts are worn.

I will report back the results.

Cheers,

Mike