Clunk on acceleration, what would you check first?

[oregonmon]

Hey all,
Short drive to run errands I started to hear a low tone clunk coming from the rear of the car towards the bottom.
It happens when starting from a dead stop and it occurs just as start to accelerate. Occasionally, I noticed it after shifting to 2nd gear.
Would it be maybe motor or transmission mounts? What do you guys think?

[kqw]

Check your CV joints
Check your Rear Shocks (tower)

[haycait911]

broken engine/trans mount would be my first check. 914's were classic for this.

[Joe Bob]

Rear sway bar mounts....

[dd74]

What Mike says. I had the same clunk. It was a broken sway mount.

[john walker's workshop]

trans hitting sway bar? should be a witness mark if it does.

[oregonmon]

So given that I'm a complete car mechanic novice, could someone show a diagram or pic of what Mike dd74 are referring to? Or would you refer to a replacement part?

[Won]

Check for rotational play in CV joints by putting the car in gear and jacking up one side at a time, turning the wheel in the air. I had clunky noise coming from the transmission area with every clutch engagement that got progressively worse, and turned out to be this: Half shaft splines worn out

Hopefully it's not CV related but it's still a good idea to check them every now and then (now I know!).

[William Miller]

Beer Bottle under the seat?
_ _ it Happens!

Wouldn't be the 1st time!

[Joe Bob]

Originally Posted by oregonmon

So given that I'm a complete car mechanic novice, could someone show a diagram or pic of what Mike dd74 are referring to? Or would you refer to a replacement part?

Print out the suggestions....

Get it up on a rack, buy a six pack, light a ciggy, ask your buddy to help ya wiggle stuff and scratch and sniff, and hopefully it will be obvious.

OR

Simply take it to a shop that does "furrin cars" and has a policy of free diagnosis....or least will quote a fee for looking.

[Ole Petter]

Faulty rear sway bar mount. I had the same...

[Superman]

The first part of the investigation should be along the lines of the beer bottle under the seat thing. Baseball. Whatever. We leap at complicated explanations when more often than not, it's something very simple.

Another comment I would make is to never, ever, dismiss JW's advice. He's been wrenching on 911's, and only 911's, for decades. He says look to see if the tranny has been touching the sway bar. That is the first thing I would look at when the car is raised (and the beer bottle is opened and some of the beer has come out).

Next I would check the sway bar mounts. I think the CV joint thing is a longshot.

[livi]

I know a broken sway bar mount are likely to produce a similar sound - but why only during accelleration? Movement from decelleration, turning and particularly road bumps would also produce a 'clunc'. No?

[ZoomGlide125]

Originally Posted by livi

I know a broken sway bar mount are likely to produce a similar sound - but why only during accelleration? Movement from decelleration, turning and particularly road bumps would also produce a 'clunc'. No?

rear sway bar i would bet my lunch on it. but just because you hear the sound during accel. doesnt mean you have to hear it during braking or turning. i had a problem with my boxster s' front motor mount, and it would only make that wierd "clunking" noise 30% of the time more or less.

[KCinBR]

Here is a pic of my sway bar mount repair, I used the factory type mounts, there are better ones one the market, but I went cheap.

Before: View from the rear wheel well



After: View from under the car

[oregonmon]

Did you notice a difference in handling or the way the car acted when you got this fixed?

[KCinBR]

Originally Posted by oregonmon

Did you notice a difference in handling or the way the car acted when you got this fixed?

OH YES..........much less body roll which I had thought was caused by my 15" Pirelli tires.

This is really a easy repair if you have welding skills and the right tools.

[MBEngineering]

HI oregonmon
I see you have a "1979 911 SC ", do you know if the clutch has been changed??? as the early SC's had a rubber center center/spinner plate fitted and broke away, which would give you the noise on acc/de-acc, as the stops hits, as in the bottom photo.
This plate is a turbo one and the SC is the same design but smaller, the rubber fractures through, as in the photo below,



and the center then spins and the stops hit, as in the photo below??



only a thought/possibility, just ignore it if the clutch has been changed.

regards mike

[oregonmon]

Clutch changed this year, but thanks for the input.

[gblumenstock]

I think I figured this out after a few years of hearing the mysterious clunk. I think its the AAR? diaphragm (large round thing at the right rear of the engine) hitting the rear shock tower under certain conditions (speed bumps, etc) . Maybe this suggests worn motor mounts? To test, try shaking the car back and forth to see if you get get the same knocking noise.

GB