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Popping in the front...

96 XJ Sport

NAXJA Forum User
Location
Bardstown, KY
I tried searching, but there can be so many reasons for popping sounds. So let me describe mine...when I put it in reverse, and ONLY with my foot on the brake I get one good POP! Then I can continue backing up putting foot on and off the brake with no more popping. After I get the one pop in reverse, if I stop and go into D with foot on the brake, it sometimes makes a pop in that direction, too, but not quite as loud. With foot off the brake, I go between R and D with no pops at all.

So here's the kicker...I first assumed that I had a worn caliper and even inspected them both and found that the driver's side could be wiggled pretty good by hand compared to the pass side which was pretty tight. Tried new caliper pins to no avail. So I did a full front brake job with calipers, rotors and pads. NO CHANGE!!! Now I'm scratching my head. What else could this be?

Eric
96 4.0 AW4 D30 open 8.25 open RE 3.5" 31s
 
maybe worn/loose U-joints? Can you have someone else listen from outside while you do this and maybe localize the source of the sound a bit more?
 
You know, I had kind of ruled out u-joints because it only happens with the brakes engaged, but perhaps I did so prematurely. I guess I can see how putting it in gear with the brakes disengaged is possibly just not enough pressure on the joint to make it pop. I will lay under it tonight and let my wife put it in R :eek:

Eric
 
This is a sign that your drive shaft ujoints are bad. If they have alot of slop it alows the driveshaft to spin slightly before hitting. being that forward and reverse will put the ujoint at exact oposite ends of its limit. It gives it the most distance to speed up and pop on the other side.

Grab your read drive shaft and rotate it back and forth if there movement... tada

Edit: I wouldn' suggest getting under the vehicle when she puts it in reverse thats asking for trouble nothing you cant hear squating from the side.
 
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I will lay under it tonight and let my wife put it in R :eek:

Eric
I hope you haven't pissed her off lately. Kidding aside, the suggestion for the driveshaft u-joints is a good one. Have you ruled out control arm bushings? I don't mean to start you on a wild goose chase, but depending on how the "pop" sounds, that might be a possibility.
 
I would definetly not rule out U-joints...check your transmission/trans case and motor mounts too.
 
Thanks for the suggestions guys. I will check the rear DS although it really sounds like it's coming from the front. And the rear DS is new (2,000 miles ago) double-cardan from DPG that I installed with the lift and RE HnT SYE.

Assuming that it is coming from the front, which I will confirm 100% tonight, then it shouldn't be front DS u-joints or axle u-joints, correct? ...because they don't have any stress on them in 2WD. I had not thought of the control arms...I also installed them with the lift. They are the RE superflex fixed length arms and I torqued to something like 110 or 120 ft lbs, but they certainly could have worked loose.

Eric
 
I would think your front ujoints would only vibe in 2wd at speed. If they popped like that they would have to be insanely bad. which you could probably feel by hand.

I still recomend trying to spin the ds with your hand. A jeep I bought with a SYE had bad slop in the tcase. In the least your just narrowing things down with 10 seconds of effort.
 
Well, she had her chance, but she didn't run over me :thumbup: I laid underneath and she went between R and D several times, but the pop was just not distinct like it is sitting in the jeep. Actually, I could hardly hear anything. Then I said, move and let me try, and I hopped up there and POP, there it is. So now I'm totally confused, lol. It's like it transmits through the floor inside the cabin.

I did check the rear DS by twisting it back and forth by hand and it's tight. Control arms did not appear to have any slack in them either. I did notice that the transfer case had a fair amount of flex in it when either R or D was engaged. Is that normal? And does that have anything to do with the tranny mount being bad possibly? I'm at 142K and I'm sure it's original as well as the motor mounts, but I don't just want to replace things willy nilly.

Eric
 
With the brakes on? nothings moving except drivetrain
 
With the brakes on? nothings moving except drivetrain

Yes, with the brakes on, so I had ruled out suspension, too. I guess in 2WD, even with the brakes on, the REAR suspension has some load on it as the DS tries to twist on the axle. So I guess I should double check the leaf spring bolts. If nothing else, I am learning to not rule out anything on a jeep!

What does anyone think about the TC flex and tranny mount question that I asked a few posts up?
 
How do the Motor mounts look? Did you watch them with the hood open when you hit the gas? They are pretty close to your feet when your driving and can transmit the pop pretty good.

My guess is Control Arm or Motor Mount.
 
I got a new trans mount from Qtec for $20 bucks, even though mine didn't look too bad- I was replacing the motor mounts as well. Side by side, the new mount was almost an inch taller, and a good deal stiffer. Replacing it got ride of one more clunk. ~143K on mine at the time. I look at it as cheap insurance, and it was a cinch to change.

Check how your front pads sit on the guide rails, and check the "slides". I finally narrowed down an odd noise up front to the rails being worn, allowing the pads to bang around when coming to a stop, or changing direction with the brakes applied. Bought some different pads that included shims, which seems to have fixed it. Something else for you to check, if you can't find a culprit in the driveline.
 
Good advice guys, thanks. I will eyeball the motor mounts tonight and the trans mount as well. If they look bad, I'm leaning toward just upgrading to some Brown Dogs and a fresh tranny mount at the same time.
 
The motor mounts look like they have good solid rubber still intact. I found some pictures of destroyed motor mounts and mine definitely don't look like that. The tranny mount I can't tell by looking, but I'm going to go ahead and replace it and see what happens.
 
Well, the saga continues, lol. To anyone who is still watching at this point, thank you :laugh3: I still plan on replacing the tranny mount regardless, but haven't had a chance yet. Anyway, I drove the XJ to work today and got to hear the pop many, many times. It's coming from the front or I've got poop for ears.

I'm starting to get frustrated, so I ask my wife to help again. She's shifting back and forth again and I'm standing outside listening. Still can't isolate the sound as it seems faint when she's doing it. Then had her keep it in R and just inch back gradually going on and off the brakes...WHOA, then I saw something unusual. The drivers' side wheel/tire appears to have slop in it. Probably not the best way to describe it...the whole wheel/tire is shifting front to back and vice versa as she's tapping the brakes.

So am I right in saying that it's one of these two things: 1) the whole axle is shifting which would point to loose control arms, or 2) just the knuckle is shifting due to worn ball joints?
 
i had a pop just like that. it was almost impossible to pinpoint too, and i could never hear anything significant underneath. turns out it was the bolts on the tranny mount, they were ever so slightly loose. loose just enough that the torque going from forward to reverse or vise versa was enough for the tranny to move a little bit, creating a pop that resonated through the unibody(which is why it could be heard so easily inside). i got under, tightened them up real quick and i haven't heard a noise since.

i found it so funny because i was racking my brain for almost a year trying to figure out what the hell it was, having thoughts of some part on its last legs snapping off on the freeway and kaboom:). it turned out to be something so incredibly simple.

don't know if this is the cause of your pop, but check it out.
 
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