Thump Bump Clunk When Accelerating While Coasting

[denaliguy]

I've searched in a lot of forums trying to find an answer for my scenario. While coasting (typically above 40mph), if I accelerate hard there is a very noticeable thump then the car accelerates as normal. The harder I accelerate the harder the thump. If I'm easy on the gas pedal is doesn't happen, or if it does it's much softer. I've got a 2008 Escalade with no known issues. The thump does not occur at stops or shifting. Only when I accelerate hard after I'm already moving down the road.

My initial guess is that it's something with the driveshaft. I read about the slip yoke issue but everyone reports the thump (or bump) when they come to a stop or accelerate from a stop. Mine does not do that. Could that still be the issue? I have some high friction, high temp valvoline joint grease that I used on my Envoy when that had the slip yoke issue. I'll give that a try, but like I said the symptoms are not the same.

Any suggestions are welcome.

 

[The_Roadie]

Guaranteed it's not the slip yoke. The way that causes noise at a stop is by encouraging stickiness in the position of the rear diff when you come to a stop, when the torque of the brakes twists the differential and changes the length of the driveshaft a tiny bit.

What you have is driveline lash (looseness) somewhere. The typical cause is U-joints, as you figured, in the driveshaft. But it could also be ring & pinion gear looseness, differential bearings, or even (my second prediction) loose control arm mounting bolts or bushings.

On trailvoy one member reported that sort of thing, and he had a CRACKED and broken rear axle control arm. There are four arms, eight mounting points (plus another pair of mounting points for the left/right axle positioning Panhard bar) I'd be looking at very closely as well as the u-joints.

 

[denaliguy]

Thanks Roadie, as always you have some great input. One thing I forgot to mention is that it feels like it's coming from the front end. I realize vibrations/thumps can travel through the driveshaft, but I would swear it's coming from the front. I'll check all of it out though, see if anything has play. Let's hope it's a u-joint issue, cheap and easy fix.

 

[The_Roadie]

And again I get caught not checking what forum I'm even in. You may have a totally different rear axle mounting than the GMT360.

 

[denaliguy]

Bump

 

[gmcman]

Have you checked your motor mounts, transmission mounts? Put in gear and while on the brake, raise the RPM's and see if you have excessive movement. Should lift about an inch, also try reverse.

 

[Texan]

Does your Escalade have a two piece drive shaft with a third u-joint?
There is a bearing and support for that third u-joint. Might be the problem.

 

[KNBlazer]

Have you checked your motor mounts, transmission mounts? Put in gear and while on the brake, raise the RPM's and see if you have excessive movement. Should lift about an inch, also try reverse.

I also, would suspect mounts....

 

[denaliguy]

Have you checked your motor mounts, transmission mounts? Put in gear and while on the brake, raise the RPM's and see if you have excessive movement. Should lift about an inch, also try reverse.

I did this test and no excessive movement. The left motor mount was replaced not too long before I purchased it.

Does your Escalade have a two piece drive shaft with a third u-joint?
There is a bearing and support for that third u-joint. Might be the problem.

It does have a two piece drive shaft. Where would the third u-joint be located?

 

[HARDTRAILZ]

Tranny mount fixed mine

 

[Texan]

I am not up to date on the full size SUV, but 10-15 years ago the Suburban had a
intermediate U-Joint with bearing support on a cross member half way between the
transmission and rear differential.

 

[denaliguy]

Tranny mount fixed mine

HARDTRAILZ, did you have the same symptoms I described in my first post? Did you fix it yourself or have someone else do it? I've never messed with a tranny mount.

I am not up to date on the full size SUV, but 10-15 years ago the Suburban had a
intermediate U-Joint with bearing support on a cross member half way between the
transmission and rear differential.

I'll have it on a lift this weekend and take a look at it. I've got a friend who's a GM Tech so we'll see what his diagnosis is.

 

[HARDTRAILZ]

Mine thumped more often but did as you described. Was a simple fix. However I never let anyone work on my truck.

 

[denaliguy]

Got it fixed, the TCM needed a software update. The transfer case was "loose." Dealer was able to replicate the thump I was experiencing and now it's gone for $79. I'll take it. Thanks for all the feedback.

 

[HARDTRAILZ]

Hmmm. That's not expected

 

[denaliguy]

When they told me over the phone that's what needed to be done I was skeptical. They said they would refund my money if I thought it didn't work. I've tried to replicate it and no thump. I don't fully understand it but I'm not going to look a gift horse in the mouth.

 

[denaliguy]

This is the most related issue I could find. Mine was not this drastic, but there was a short lag in acceleration causing the thump and then acceleration after I already hit the gas.

"In certain driving situations, there may be a three to four second lag in acceleration due to the transmission control module (TCM) programming, which could increase the risk of crash."

http://www.consumeraffairs.com/news/gm-recalls-cadillac-srxs-050114.html

 

[HARDTRAILZ]

I need to look into this myself then

 

[AlienEsky]

Did you have this problem as well? I do, I realize its an old thread but I am curious if you had this issue and fix. Thanks in advance....EDIT: Sorry, I see yours was a trans mount. Hear about anyone else with this issue?

 

[Maverick6587]

Did you have this problem as well? I do, I realize its an old thread but I am curious if you had this issue and fix. Thanks in advance....EDIT: Sorry, I see yours was a trans mount. Hear about anyone else with this issue?

You should start a new thread and link this thread in that post. Please include as much detail as possible about your vehicle's issue. As well as your vehicle model.