constant thump when coasting, need ideas

Whenever I take my foot off the gas, I start to get a rhythmic thump from the back of the Jeep. This is most noticeable at low speed with minimal wind noise but it has started to become more aggressive recently. I thought at first that something was just bouncing off the back seat or the spare tire was banging or something but it's not anything like that. Also the fact that any amount of throttle is enough to cancel it out and I'm 99% sure that it's something in the drivetrain.

Has anybody else had this problem?

The Jeep is 340k miles on it although the rear axle was replaced at some point unknown due to a collision

Start with u-joints in the driveshaft.

there is a little bit of play in the shaft but it's not at the joints

if I put the tranny and tcase in neutral there is about 1/2 inch of wrist rotation from side to side before the jeep starts moving in the direction i'm twisting

bump for the thump! driving me mad

Well if your ujoints are good that only leaves tranny/tcase/diff I know no one wants to hear that . Check your fluids for shavings.

Put on a helmet buckle up and wait till it blows lol.

tcase should be fine since it was rebuilt fairly recently

it could be the diff. there is a thunk back there when changing from forward to reverse sometimes. this jeep has 340k miles on it too, although the rear axle was replaced sometime in there due to rear end collision. I changed the fluids a few months back and did not find unexpected levels of shavings. I just checked the level last week and it was fine.

wheel bearings maybe

need ways to test this stuff

If the noise is changing with the speed of the vehical and not the speed of the motor, I would say think that is is aft of the transmission. Does the sound go away when you are on the brakes? Bad retainer spring or something in the drum rubbing? You could be so lucky.

Have you actually checked the rear universal joints? That's the most common cause of that thunking. It might well be worthwhile taking the straps off the rear ones and checking all cups for binding or tightness if there is no sign of play. It takes very little to cause noise, and especially on older ones with slightly worn yokes, it's pretty common to develop end play on the cups that are strapped down.

bigdumbfun said:

Does the sound go away when you are on the brakes?

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Yes and No, but I think you are onto something here. If I just kinda press the brakes to lightly slow down then the thump continues, but if I step into the brakes to stop then the thumping goes away. It may be that the rear brakes are not engaging until I get into the pedal though.

"rebuild brakes" is on my to-do list but I honestly have not looked at them other than to inspect pads

What should I look for

Matthew Currie said:

It might well be worthwhile taking the straps off the rear ones and checking all cups for binding or tightness if there is no sign of play.

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I'll do that when I jack it up tomorrow, thanks

ehall said:

What should I look for

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Most of it is pretty obvious: worn out, delaminated shoes, broken springs, etc. On an XJ, the other thing to look for is frozen or inoperative adjusters. Even when they're right they're so close to wrong that it's hard to tell the difference. Make sure the adjusters turn freely, the self-adjusting pawls are not worn out, and that the cables are good. If someone else has rebuilt the brakes at any time with generic parts, look also very carefully at the upper cable guide. This guide inserts into a hole in the shoe, and then the top spring goes through the hole in both. It is possible for the guide to come out of its hole, but still be held nearly in place by the spring, and when this happens, your brakes will never self adjust. It's a p***-poor design to begin with, not helped by replacement guides with too shallow a boss.

If you have an odd thumping, look also to see if there's a big rust ridge on the outside edge of the drum. Combined with axle end play this might cause some odd noises, though it would likely only happen on turns. Don't sweat a little end play in C-clip axles.

does the "thump" occur about twice for each revolution of the wheel? If so, I'd say axle/wheel bearings. Jack it up and try to move wheel up/down and fore/aft. Any play and they're bad-- also try spinning them and listen for noise. Any noise and they're bad.

loose lugnuts, drivers side!

we shall never speak of this again!

deal!! now use the time you were gonna fix it with to look at my "should I have a tach signal during cranking" thread.......

ehall said:

loose lugnuts, drivers side!

we shall never speak of this again!

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I will speak of this briefly before shutting up: if you have the Jeep alloy wheels, expect to have to retorque them after a few miles every time you change a wheel. They tend to loosen even if you torque them correctly the first time. I think it's because there are no steel inserts in the holes.