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1973 450 SL, 2002 Audi TT
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2 Posts
Discussion Starter #1
I just acquired a 1973 450SL. It was very sluggish when I got it (not driven much), I have done some tune-up work on it and engine is running pretty peppy now. However, I noticed if I accelerate hard, from a stop, there is a loud thumping sound (thump thump thump...) that comes from under the floor just behind the seats for a few seconds (until I let up a little – then it goes away). This only happens on a foot to floor acceleration and I only noticed it because I was seeing what improvement I had made on the pick-up if the engine. I know what wheel skip is and I am pretty sure it is not that. The suspension is kind of spongy and I wonder if the drive shaft gets torqued when the weight of the car shifts back… Any thoughts on how to isolate and fix?
 

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Always Remembered RIP
1983 380 SL
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112 Posts
I just acquired a 1973 450SL. It was very sluggish when I got it (not driven much), I have done some tune-up work on it and engine is running pretty peppy now. However, I noticed if I accelerate hard, from a stop, there is a loud thumping sound (thump thump thump...) that comes from under the floor just behind the seats for a few seconds (until I let up a little – then it goes away). This only happens on a foot to floor acceleration and I only noticed it because I was seeing what improvement I had made on the pick-up if the engine. I know what wheel skip is and I am pretty sure it is not that. The suspension is kind of spongy and I wonder if the drive shaft gets torqued when the weight of the car shifts back… Any thoughts on how to isolate and fix?
Did you look at your "flex disc"? Also, Please update your profile so we know who/where you are!
 

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Registered
1973 450 SL, 2002 Audi TT
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2 Posts
Discussion Starter #3
I have the records from this an it looks like the flex discs were replaced not too long ago. This car is new to me, and not had a MB before, how do you tell if they are bad?

Profile update with some basics - thanks.
 

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R/C107 Moderator
1986 560SL: '84 500SL: '84 280SL 5 speed: other 107s
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32,354 Posts
Flex discs, Engine mounts, Trans mount. Shaft carrier bearing.
Any others suggestions?
 

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Registered
1981 380SL
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256 Posts
Worthwhile to visually inspect the flex disks for cracks. I only say that because when they fail, things can get REALLY messy.

Otherwise, Rowdies list is a good one. Wear items starting from the engine and going back:

1. Engine mounts and shocks.
2. Trans Mount.
3. Forward Flex Disc.
4. Driveshaft Support Bearing.
5. Driveshaft U-Joint.
6. Rear Flex Disc.
7. Differential Mount and Rear Subframe Bushings.
8. Rear Shock Absorbers.
9. Halfshaft CV Joints (Inner and Outer). Just look for boot condition here. If you've got one that's ripped/leaking, dig further.
10. Rear stabilizer (anti-roll) bar end links.

I just did a complete suspension/drivetrain overhaul (front and rear) on mine. Just about every item listed was beyond limits. That was on a 1981 model with ~167,000.

Rides like a dream now though. No more rear end sag on accel.
 

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It Is What It Is, Dude
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22,841 Posts
I have the records from this an it looks like the flex discs were replaced not too long ago. This car is new to me, and not had a MB before, how do you tell if they are bad?


Check Section 41 of the EGv107 for a most pertinent thread.

If you have the receipt for the work done I'd suggest you visit them and see what they say. If it is related to their work they may honor the warranty if any is due.
 

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Registered
1984-380SL & 2003-SLK320
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7 Posts
This is my first MB...It's 84 sl380. I am having similar thumping sound from behind when I push the accelerator. I have changed the central shaft bearing. Also the front and rear flex disks are good. Today I changed the transmission mount. But the sound doesn't stop. I am now suspecting the U joint on shaft. Has any of the members changed the u joint.
 

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Registered
1984-380SL & 2003-SLK320
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7 Posts
Thanks for your reply. I am in Dubai and reckon I will get the U-joint rebuild. Could you let me know what exactly goes wrong with the U-Joint and something I should look for while getting this job done.
This should be a big job as need to drop down the exhaust, remove flex discs, centre bearing and then the U-joint. I think there is play in the U-Joint which is causing this thumping sound. I really appreciate your comments. Thanks
 

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Registered
1981 380SL
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256 Posts
What happens over time is the grease dries out and the bearings disintegrate (needle bearings). To inspect, you flex it and make sure it's tight without any roughness in its movements.

A bad U-joint can cause a thumping sound if it is really bad, but initially it would just be vibration at speed. When they did your center bearing, they should have inspected your U-joint. Did you do the center bearing yourself or at a shop? If at a shop, did they note a problem?
 

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1984-380SL & 2003-SLK320
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7 Posts
I got the center bearing done at a local shop and they did not bring this to my notice.There is lag in power when I shift lever in drive and then the thumping sound. I will get the U-joint rebuild and hoping that will resolve the above issues. You have mentioned that your car runs like a dream. I am just waiting for the day when my car runs like a dream. Dubai weather is hot and it rains once or twice in a year which will give me the opportunity to use this car as my daily drive.
 
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