Another rear end "thumping" mystery

[tgreen]

It is still bumping!!!! I get a thump in the rear end of my 72 coupe as I pull away from a stop. It also will do it if I let off the gas and coast before getting back into the gas (mildly, not hammering on it). It also does it if I am going along slowly, in any gear and ever so lightly give her just a bit of gas and then let off. It bumps both ways (getting on the throttle and getting off). Both rear wheel bearing assemblies have been rebuilt. I have tugged on the half shafts and seen no obvious play. The half shaft u-joints do not look too old to me but I have not had them replaced since I have owned the car (2001). I am thinking that it could be the rear driveshaft u-joint. Is there any simple, conclusive test for this? I hate to keep tearing things apart and throwing parts at a problem.

Another possibility I have considered is that there is something loose in the suspension that bumps one way when load is applied and bumps back the other way when the load is removed.

I know this type of problem has been discussed before and I recall there are some real experts here so I would appreciate any suggestions.

Thanks
Ted

 

[GTR1999]

Ted check the condition of the front differential bushing,the two piece one that bolts to the front bracket and frame. If everything else is checking out ok then this may be your problem. You're not getting a knocking noise from inside the differential right?

 

[68Roadster]

Ted,
Make sure you have the cup washers installed on either side of the strut rods where they are bolted thru at the center bracket.
The last "Bubba" that was into mine left the cup washers out and just like yours, it would thump leaving a stop sign. Without the cup washers to center the bolt, the strut rod can move around, smacking the bolt. Mine had a nice wear spot where this had been happening for years.

Cee

 

[ernie d]

Ted,

I just had the very same problem. It was the u-joints in the rear half shaft. Problem is not that they were loose and sloppy when pulling on it, but instead they were dry (needed grease badly) and were VERY difficult to articulate when rotating.

I changed out the joints (you need a u-joint press) and car is good as new again. Try it!

 

[vetteboy86]

I have the same kind of noise in my C4. For a while I thought for sure it was in the differential. However it hasn't gotten any worse. I have had this noise for about a year and a half, and it never gets worse. I had it at the shop, because I ran out of options, and they said they couldn't find it. I still think the problem might be in the differential. I replaced all the ujoints on the car. I also had the transmission serviced two months ago. Everything looked find there. Guys I have no idea, but I have almost given up. Good luck with your problems, although the guy at the shop said some noises never show up until something breaks.:cry .

Craig

 

[joe1975]

Just a shot in the dark,but could your spare tire carrier be loose.

 

[vetteboy86]

I dont know how the cars are set up, but when I looked under my car I saw the tranny moving alot. I thought it was enough to cause a noise. I know there are two motor mounts, and figured there to be one on the tail shaft of the tranny. If that was bad, then maybe it could be making a noise. Does this exist. Do the C3's have a tranny mount that could be causing some noise?

Craig

 

[C3RVETTE]

Ted check the condition of the front differential bushing,the two piece one that bolts to the front bracket and frame. If everything else is checking out ok then this may be your problem. You're not getting a knocking noise from inside the differential right?

I've replaced mine,but my thump/clunk sounds different when I'm driving...it only clunks making hard right or left turns.When sitting and the idle is up,it clunks going from out of gear into gear.But not noticeable when the idle is slow.I've replaced all of the u-joints and the rear wheel bearings too,but havent done anything with the tranny,diff. or the axle,except I switched to synthetic rear lube with additive,of course.I currently using Amsoil.I never thought about checking out the tire carrier.Everything looks like its there,but I've never lowered the tire down in the 7 months I've had the '79.

 

[mike weyman]

there is a mount on the back of the transmission .if the tail piece moves almost any at all the mount is bad.never heard of one wearing out on any vehicle though.mike

 

[tgreen]

Thanks guys. I have a few more things to check now. I guess it makes sense to try something simple first like greasing the u-joints. While I'm under there I will check differential bushings and be sure the washers are installed on the strut rod. I'll let you know what I find.


I had the car out yesterday for a nice cruise and it did not make the noise much at all. It seems to do it less and less the more the car is driven.

 

[tgreen]

Well I jacked her up tonight and crawled back under to do some more investigating. While I was under there I greased the half shaft u-joints. With the car up on jack stands and the transmission in reverse, should there be any rotational play in the driveshaft? I can twist it and get some movement. I was not able to detect any horizontal or vertical play in the driveshaft though.

The strut rod mounting looked O.K. to me. Anyways, after greasing the half shaft u-joints I let her back down and took a test drive. Still thumps. It seems to be worse for a while after it has been jacked up. Does that make any sense?

 

[C3RVETTE]

Well I jacked her up tonight and crawled back under to do some more investigating. While I was under there I greased the half shaft u-joints. With the car up on jack stands and the transmission in reverse, should there be any rotational play in the driveshaft? I can twist it and get some movement. I was not able to detect any horizontal or vertical play in the driveshaft though.

The strut rod mounting looked O.K. to me. Anyways, after greasing the half shaft u-joints I let her back down and took a test drive. Still thumps. It seems to be worse for a while after it has been jacked up. Does that make any sense?

Hoping for an 'expert' opinion myself,as you're going beyond any mechanical knowledge I might have.But would jacking loosen things up with the wheels unsupported? How about play in the halfshafts-back & forth,and rotational?

 

[GTR1999]

Look at the caps on the U joints. If you see rust dust, then they are dry and the needles are worn. There should be no play in the joints. Don't try running it in gear with the rear jacked up unless the trailing arms are in the normal height or the ujoints will bind. The original joints were zerkless so someone has changed them in the past,could one of the zerks be hitting on the yoke?

Usually the front differential bushing is the problem. It is a 2 piece bushing and the top portion is what you can't see.I'd take a closer look.

 

[81 Corvette]

Mine was making some thumping as well some time back. My ujoints were toast, and you could move them up and down about 1/8-1/4 in. Make sure you also check the in/out play on the half shafts...that is, can you pull them away from teh diff or push them into the diff.

here is one of my ujoint caps

and my diff carrier pin

i'll just conclude with, thumping is probally one of the worst noise you wanna hear, or at least in my case it was.

 

[C3RVETTE]

My u-joints are all new Spicers,and my diff.mount has been replaced with the poly version,all in the last 9 months.Looks like one half shaft needs replacing.What else could move side to side,something in the differential?

 

[dwilson50]

There is a bracket that extends forward from the dif housing to above the rear u-joint. On the front top of the bracket is a rubber "biscut" that is there to absorb the torque rotation of the differential. If that "biscut" is shot...you get a thump on takeoff and decelleration.

 

[GTR1999]

1/2 shaft in/out play is caused by worn differential yokes or completely worn out U-joints/joint yokes.

The differential yokes wear against the center posi pin that is shown in the picture. The best way to tell how much play you have is to measure it with the 1/2 out or at ride height. You want less then .050" if you find you have .070" + you're going to need new yokes. If they are the orginals then that may be your problem. From the sound of the original post it did sound like the front bushinig though. The only way to replace the yokes is to remove the differential from the car.

 

[tgreen]

I drove her in to work today for the first time this year. The rear end noise is not getting any worse (it may even be thumping less frequently the more I drive it) but now the tachometer quit working. Its always something. This make me think about trading for a newer, more reliable vette. Maybe an early C5. But my chrome bumbers sure do look sweet.

 

[JohnZ]

I'd bet on the diff nose bracket bushings as the cause - you have classic bushing deterioration symptoms. If that's the case and you replace them, avoid the aftermarket "poly" replacements - they're all made wrong (too thin, so they raise the diff nose too close to the floor pan and screw up the rear U-joint operating angle) and don't have the correct internal spacer to keep them concentric. Get the correct OEM rubber replacements with the molded-in centering spacer, and you'll be good for another 40 years.

 

[GTR1999]

I agree with John,look over those bushings again and use rubber. I never use poly except for motor mounts-had good luck with them.

The tach is driven by a cable from the distributor. May be something simple as a loose cabel but don't count on it.Remove teh distributor end of the cable and spin it the tach should move. If it doesn't the cable may be broken in the casing. If it does then you have to look into the distributor for a worn side gear and main shaft gear. If the side gear is worn then the main shaft is most likely too. The after market gears are a crap shoot for quaility. I had rebuild a couple of distributors last year and had a problem with the available parts. Look into High tech in Fl they make their own gears and do it right.

Lastly, realize you have a 32 year old car that may have been worked on by any number of "mechanics" that in conjunction with age will cause things to stop working. Believe me you're not alone!