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Drivetrain Knock

jeepn_4_life

NAXJA Forum User
Location
Washington
I have a 94 flame red xj that I absolutely love and plan on driving forever, but it is only just getting broken in at 175,000 miles and over the past 10,000 or so there has been a knocking coming from the drivetrain somewhere.

Basically, when I am off the power and when it is coasting and gets slow enough (10mph and under) it starts knocking. I changed the U-joints in the rear shaft a while back and the bearings in one of them had turned to dust, but it didn't help. I have been under there and twisted the front one by hand and it doesn't seem loose, but the knock becomes subdued in 4wd. Anyway, not knowing anything about anything, I thought it might have to with the transfer case and maybe a chain replacement or a completely new t-case (which probably needs to be done anyway). Anyway, I would appreciate any thing you guys may have for advice.

I have caught the urge to modify my jeep from reading about your rigs. Right now, it is completely stock and the water pump is the only thing I have had to replace in 175k, so I am pretty impressed. I guess my short list for mods are 2.5-3 in lift, winch bumper, top rack and lights. Anyway, jeep on.
 
How are your motor mounts? Tranny Mounts? O and welcome to NAXJA!:wave:
 
jeepn_4_life said:
I have a 94 flame red xj that I absolutely love and plan on driving forever, but it is only just getting broken in at 175,000 miles and over the past 10,000 or so there has been a knocking coming from the drivetrain somewhere.

Basically, when I am off the power and when it is coasting and gets slow enough (10mph and under) it starts knocking. I changed the U-joints in the rear shaft a while back and the bearings in one of them had turned to dust, but it didn't help. I have been under there and twisted the front one by hand and it doesn't seem loose, but the knock becomes subdued in 4wd. Anyway, not knowing anything about anything, I thought it might have to with the transfer case and maybe a chain replacement or a completely new t-case (which probably needs to be done anyway). Anyway, I would appreciate any thing you guys may have for advice.

I have caught the urge to modify my jeep from reading about your rigs. Right now, it is completely stock and the water pump is the only thing I have had to replace in 175k, so I am pretty impressed. I guess my short list for mods are 2.5-3 in lift, winch bumper, top rack and lights. Anyway, jeep on.

I currently have the same noise coming from my '92

Does your knocking go away during acceleration and come back right when you let off the gas?

Mine is knocking pretty bad, speed does not matter
 
Yeah, it definitely goes away during acceleration and while I am at speed. It seems to be only during coasting and in order to keep it from happening as much I usually downshift through the gears to keep pressure on system. It is definitely more prevalent if I put the thing in neutral and coast to a stop. I would like to figure it out because it is annoying.

I have to tell people who haven't ridden with me before that "I am about 85% sure that the driveshaft is not going to fall off...." :dunno:
 
I had something similar happen to me, I knock at low speeds but nothing at anything over 10mph. It turned out that there was a small rock inside my tire and when I would be going slow enough it would just roll around and bounce off the wheel! I must have lost the bead and not seen the little rock inside before I got the tire back on the wheel. Welcome to NAXJA!
 
I had the exact same thing and it turned out to be a warped rotor (or maybe the caliper was sticking???) and it was causing the caliper to knock up against the carrier. I too thought it was a u-joint and changed the rear ones. Try pulling off the rotor and wiggle the caliper with some force and cycle speed as the sound you hear. It makes a deep hollow kind of knock.

I only heard the sound when coasting up to a stop light for maybe the last 5 or 10 feet. "knock, knock, knock" then quiet.
 
i have the exact same noise coming from my 99 with 54,000 miles and 3.5 inchs of lift. i changed all the u joints and think it might be the slip yoke that is making the noise? i can get under the jeep and wiggle it around so maybe check that on yours. i put some grease on the slip yoke and it stop for about 30 miles but then it came back. if not keep me updated on what was wrong and making the noise. thanks
 
If its an A/T I would automatically check the flexplate. They are notorious for loosening up and cracking with a bunch of different sounds possible, most noteable is a knocking, you could almost mistake it for a rod bearing.
 
dizzymac said:
If its an A/T I would automatically check the flexplate. They are notorious for loosening up and cracking with a bunch of different sounds possible, most noteable is a knocking, you could almost mistake it for a rod bearing.

Ok, I definitely don't have any idea what a flexplate is, an internal transmission part?
 
On a standard trans it would be similiar to the flywheel, it has the ring gear on it for the starter drive to engage. A flexplate is much thinner and lighter than a flywheel. The TC bolts to it.
The bolts on these have a habit of loosening up and the plates have a habit of cracking,(making them really flex), you can find a lot of info on this in "search"
Another symptom that has been reported ( I can attest to this) along with the knocking noise, is a problem trying to back up...it seems that the plate can flex so much that CPS has a hard time operating correctly and causing the eng to stall in reverse or "bog out" when trying to give it gas.
 
sounds like he has a manual trans.How rapid are the knocks?Are they light or heavy?Try to move the slip yoke up and down to check for play.Are the front axle u-joints good?

Remove the front driveshaft and see if the noise goes away.....
 
mine started doing this today, only when i'm braking thou. It is most definately in the drive line, because if I put it in N and apply the brakes it doesn't do it.

I'm hoping its something simple as I'm kinda tapped out right now.
 
I have a manual trans, but to answer your questions. The knocks vary in how rapid they are with speed. Slower speed slower knocks, faster speed faster knocks. They are decently loud (enough to cause people to worry...) but other than that I don't know. Good advice about the front drive shaft, I might remove that and see what happens. I will also have to check out the slip-yoke. Unfortunately for troubleshooting purposes, my jeep is back in washington state and I am in Washington DC for three months with work but it was something I wanted to gather info on it now so I can fix it before I take it out this summer doing all the fun stuff. I really appreciate all your replies though, so thanks.
 
Found out what the knocking was...

The rear passenger side wheel had lost 2 lugs, 2 where hand tight, 1 was secure!

Doh!
 
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