Need some help from the jeep guys

[twisted]

i have a 2001 jeep Cherokee sport.it has 62 k on it and is always serviced.4.0 engine and 4 speed auto tranny. the problem is when i come to a stop i feel a thud and then again when i take off.not all the time but alot.

looked for bad u joints, tranny mount and the breaks. found a bad caliper up front and thought that was it but isnt.plus when i go down the road and i put into neutral it wont do it so that rules out the brakes.

my local mechanic said i was feeling the backlash in the 4wd system.

ive read that others with a cherokee have felt the something but i can not find an answer to why.

i know alot of you guys are into jeeps so i thought i would ask here.

thanks !

[JeepinOR]

Where are you feeling the thud? Can you hear it coming from a specific location?
Is the thud consistent at certain speeds/RPM, like when the transmission is shifting, perhaps?
Have you changed anything within the last several thousand miles?

[01XJ]

What are the specs on your jeep any lift/mods or just stock?

[Destroyer]

Check your motor mounts and exhaust routing. My exhaust is got pushed up just after the crossmember and since there is not a flex joint designed into the system, it hits the floor when the engine moves under load.

[twisted]

the jeep is stock (for now). nothing has been changed in the last 1000 miles. the thud sounds like its in the rear of the jeep.it only does it on breaking rite before the complete stop(you come to a stop sing,hit the break just about stopped and thud) and when you start going rite from a dead stop(you take your foot off the brake and hit the gas and thud). if you let of the break and roll it wont do it on starts.also its only one "thud"

[Offroader5]

Does it feel at all like it's coming through the floor? Check both your front control arm bushings & leaf spring eye bushings. Sometimes when these start to go bad...you won't so much feel the thud when you hit a bump but when the body moves fore and aft like when braking or taking off, the slop in the bushings can sound like a thud.

If you have enough backlash in your stock r&p to feel a thud....it's past the point of needing some TLC. I don't think it's the "backlash in the 4wd".

[rock_krawler]

My '99 does the same thing, it is lifted though. The way it was explained to me was that the rear axle twists forward during a stop. The axle loads the leaf springs and the brakes hold it "loaded". When you take off or release the brakes, the springs force the axle to rock back causing the thump. I hope this makes sense. I believe this is what the condition is that you described.

[2JSC]

What transfercase does your Jeep have? If its the full-time 4WD version, it could be the coupler inside.

[xgerstandtx]

np249(alltime 4wd) in grands xjs are either np242 or np231 or even early np207. when viscous couplers go bad they will not reflect that on straight linedriving or breaking. most likely tcase is not issue. Check all CA bushings and rear spring bushings, motor mnts and exhaust routing than go find a new mechanic

[JeremyH]

Originally Posted by Offroader5

If you have enough backlash in your stock r&p to feel a thud....it's past the point of needing some TLC. I don't think it's the "backlash in the 4wd".

Agreed! Dont go back to that mechanic....

Like it was said above, check the control arm bushings. Also check the track bar and it's bushings. Those track bars have been known to come loose and will make a sound like what you describe when accelerating and decelerating.

[Offroader5]

Originally Posted by rock_krawler

My '99 does the same thing, it is lifted though. The way it was explained to me was that the rear axle twists forward during a stop. The axle loads the leaf springs and the brakes hold it "loaded". When you take off or release the brakes, the springs force the axle to rock back causing the thump. I hope this makes sense. I believe this is what the condition is that you described.

Mine is lifted a bunch and I've never had the problem you're describing. Only way that would happen is if you were getting alot of axle wrap...and that could be caused by using stock leaf packs with lift blocks. Still would only wrap the axle and not really create a thud noise. It'll only thud if there's something worn or loose.

Could also happen on a rig with wore out leaf packs that are flattened. When they flatten out, it pivots the shackle back and the shackle may be hitting the sheetmetal rear face of the "box" that the shackle is mounted in. This is why they make boomerang shackles for the XJ's...to give more clearance for the shackle to rotate back.

[Offroader5]

Originally Posted by xgerstandtx

np249(alltime 4wd) in grands xjs are either np242 or np231 or even early np207. when viscous couplers go bad they will not reflect that on straight linedriving or breaking. most likely tcase is not issue. Check all CA bushings and rear spring bushings, motor mnts and exhaust routing than go find a new mechanic

Being the sport trim level in that year...it's more than likely a 231...if it's the original tcase still in it. The 242's were only used in higher trim levels like the limiteds & wagoneers. The 207's were only used in '86 and some early '87.

[twisted]

my leafs are pretty flat and the rear end sits a tad lower then the front. im not sure what transfer case is in it.
so list to check:
front control arm bushings
trac bar
rear leaf eye bushings
exhaust mounting
motor mounts

this may be a stupid question but hey i wont know till i ask. will it be obvious if parts are worn out ?should the jeep put on a lift to see if the above parts have any play ?

so it sounds like the drive line is ok or atleast is not the problem (i know you can not say 100%) ? i dont have the knowledge or the proper tools to check that sort of thing.

i appreciate all of your guys help. ill try to get some pics, maybe that wil give you guys a better idea.

Geo

[run2jeepn]

This maybe stupid, but check you U-joints in the rear. If one is that bad it can make a thud. It should just break... But I've had it happen...


so you don't know if the sound if coming from the front or rear of the Jeep?


Edit: removed the treack bar comment. Thought XJ's had rear track bars like YJ's...

[Offroader5]

Originally Posted by run2jeepn

But I'd guess it's a Bushing.. If it's a Trackbar Bushing. Just take the whole Track bar off. You won't miss it.

Whoa....whoa...whoa. The trackbar? I would certainly not remove the trackbar. Are you sure you're not thinking of the swaybar? The front suspension links are not triangulated enough to do without the trackbar. If there was no trackbar, everytime you would turned the steering wheel it would push or pull the whole rig on top of the axle back and forth. Not to mention the extreme possibility of death wobble.

[bigflex]

did you leave a watermelon or something in the back? That would cause a thud sound as it rolls forward when you stop and backward when you go.

sorry I just couldn't help myself.

[JeremyH]

Originally Posted by Offroader5

Are you sure you're not thinking of the swaybar?

Or traction bar?

[wonder squirrel]

This may sound crazy, but it is most likly the driveshaft. Pull the driveshaft off and lubricate the yokes splines really well (where it slides into the tailhousing) with some grease and put it back in.

[CRSMacgyver]

Originally Posted by run2jeepn

But I'd guess it's a Bushing.. If it's a Trackbar Bushing. Just take the whole Track bar off. You won't miss it.

Definitely DO NOT DO THIS! This is not a leaf sprung YJ/CJ that you can remove the trackbar on. If you try and drive an XJ without a trackbar(and with stock style suspension) it will collapse. You must have it installed.

Check the u-joints on the rear driveshaft. I would put money that it is something in the rear driveline. How many miles are on it? Depending on what rear axle it has and whether or not the fluid has been changed in it that may be your issue as well. If the bearings have worn significantly there could be excessive backlash in the rear gear setup that will cause the exact noise you are describing. Pull the fill plug on the rear diff and take a small sample of the fluid. If it is extremely black or has a green metallic color to it it could be bearing in the rear axle. If you look at it and want to discuss it further I will be glad to see if I can help. Owned a 4x4 shop for a bunch of years. PM me.

[JeremyH]

Originally Posted by CRSMacgyver

Definitely DO NOT DO THIS! This is not a leaf sprung YJ/CJ that you can remove the trackbar on. If you try and drive an XJ without a trackbar(and with stock style suspension) it will collapse. You must have it installed.

Ummm...last I checked, XJ's had rear leafs.

BTW, if you remove a track bar, the vehicle wont "collapse"...the axle simply wont track straight. There is not enough triangulation in the uppers and lowers to keep the axle from moving side to side. I've removed the track bar on my TJ plenty of times and it never "collapsed"....I've never tried to drive without one though...