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Unread 07-25-2015, 12:22 PM   #1
Rwrangler
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1991 YJ Wrangler 
 
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Need help with diagnosing vibration problem YJ

It seems to have a mind of it's own. Some days the YJ runs like new, other days I could trade it in. Any how. Mine is a 5spd manual (see vehicle info below). Some times when I start out in 1st gear I get a shudder, as if the clutch is bad. And if you use reverse gear it's also real bad. But when your moving and get past the intial clutch engagement everything is real smooth? This has been going on and off for the last 8 years. I first thought it was the clutch. I noticed a oil leak on the bottom of the bell housing and had it check, the shop said it was a rear main seal. So I thought the clutch had a little oil on it. So I had the shop pull my clutch. Clutch disc was perfect, no oil at all and the t-brg wasn't leaking.. But sense the clutch assembly was out I had them replace with all new parts, clean the flywheel, replace the pilot brg, clutch disc, pressure plate and t-brg. No change at all. Everything looks good underneath, U-joints are all good, no slop in trans/transfer/diffs. Could it be a problem with the rear limited slip diff? I do service the rear diff every year and add the little tube of additive. It seems to be coming from the rear?

1991 Jeep Wrangle Renegade, A/c, Hard top
Engine 4.0 liters MFI 6 cyl 258 cubic inches
Transmission Manual 5 speed: AX15 ???
Transfer Case: NP231 ???
Front differential: Dana 30
Rear Differential Limited Slip: Dana 35, 355 gears
Tire Size: BFG AT 30x9.50 R15LT

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Unread 07-25-2015, 01:02 PM   #2
Coyotess
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1995 YJ Wrangler 
 
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Location: Kennewick, Washington
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I just did the undercarriage hunt for a "clunk" and it had an accompanying shudder at times. It was my front yoke. I looked at it several times and did not feel or "see" the issue with it. Had to get it at the right time to find the culprit.

Check it and your u-joints/bolts. Any give there will cause all sorts of odd things. Clinks, clanks and clunks as well as shimmies and shudders.

I just dealt with this and am gearing to fix it all. Good luck with your hunt!
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Unread 07-25-2015, 01:52 PM   #3
Rwrangler
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I don't have any clunks. Just the shutter when the initial clutch engages (sometimes).
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Unread 07-25-2015, 03:35 PM   #4
mark1305
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Most of my experiences (from the 60s onward) have been that clutch discs actually have a wavy surface to prevent shuddering as they engage. Even if enough material is left to pass specs, if they have worn perfectly flat they will tend to shudder. YMMV. I've never seen the specs for how wavy the pressure plate material is ground, but its not very much.
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Unread 07-28-2015, 05:41 PM   #5
Rwrangler
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mark1305, I would have to rule out the clutch assembly. When I thought the liberation was the clutch assembly the first time, everything that was removed looked new, hardly any wear, not oil soaked, but because it was all apart I had them install all new parts. So all in all over the years, not being able to figure out what the vibration was, the clutch assembly has been completely change out with all new parts 3 times (unnecessary all three times). And like i stated before, underneath looks good. I've check all the U joint, yokes, pinion brg, slip joint, trans mt is new, all tight. The only things I can think of is the motor mts, transmission, transfer case or Limited slip differential? But again there is no noticeable defects there? If it happened all the time it would be easier to diagnose but you never know when it will act up? It does seem it gets worse after driving a long time and in hot temperatures. I do a complete oil and lube change every year on everything. The engine every 3000 miles. This has been happening off and on for 10 years. I hope someone out there can shed some light about this problem.
I'm stumped
Note?
I serviced the rear diff:
Differential (rear) Oil: Castro l75-90 Synthetic limited Slip Oil #6674
Differential (rear) Oil Additive: K&W/7 oz. Posi-Trac additive #2508 CHANGED ON 11-29-2014
Manuel Transmission Oil: Castro 75-90 Synthetic limited Slip Oil #6674. WAS FULL @ LAST SERVICE 11-29-2014
Differential (front) Oil: Castro l75-90 Synthetic limited Slip Oil #6674. WAS LOW @ LAST SERVICE 11-29-2014
Transfer Case Oil: Dexron II. WAS LOW, but no seal leak? @ LAST SERVICE 11-29-2014
Lube job: DONE ON 11-29-2014
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Unread 07-28-2015, 06:55 PM   #6
mark1305
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Good luck with it. Intermittent gremlins are the hardest to sort out, by far.
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Unread 07-28-2015, 07:13 PM   #7
ROWA
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Rwrangler,
Do you notice a difference if you take off slow or fast. Your symptoms could be axle wrap, in first gear when you get hard on the gas your rear axle pinion tends to rotate up taking the driveshaft out of alignment, this can cause vibrations.
Just a thought,
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Unread 07-28-2015, 07:17 PM   #8
Rwrangler
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Actually, I try not to give gas at all. I try to let the clutch out very slow. And then if I have to give it a small amount of gas or it will feel like the rear of the Jeep will fall out. Then other days it won't happen??
But yes that's what it feels like. But if I remember if I give it more gas and let the clutch out fast, it's not so bad. It seems only when the first engagement of the clutch. Once the clutch is engage, even with no gas, you can step on the gas hard and no vibration. Even if I'm just rolling very slow it doesn't happen. Could my spring or bushings be worn out??
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Unread 07-28-2015, 09:12 PM   #9
Michaelgoesrawr
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Subd for info
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Unread 07-29-2015, 04:54 AM   #10
glenn-91YJ
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I'll just throw this out there...Could it possibly be a rear brake cylinder sticking where it isn't releasing completely and causing the rear end to shutter/jump. It would be most noticeable in first when starting out.
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Unread 07-29-2015, 06:51 AM   #11
Rwrangler
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How would I diagnose that? You might have something there? Obviously it happens after I stop to go forward or backwards, and it doesn't mater if I'm going straight or turning, and it only happens in first gear. It's bad when it happens. It's like you have a real real bad clutch. Shakes the Jeep violently. Then sometimes it won't do all day? That's why I thought it might be the limited slip diff? But the brake sticking would do the same thing? If I'm on a slight down hill and stop for a red light, and release the foot brake to go before I engage the clutch the Jeep will roll, so if the brakes were locking wouldn't I feel that then??
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Unread 07-29-2015, 07:14 AM   #12
Michaelgoesrawr
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Unlikely you would feel just one wheel drag.
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Unread 07-29-2015, 07:16 AM   #13
glenn-91YJ
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Quote:
Originally Posted by Rwrangler View Post
How would I diagnose that? You might have something there? Obviously it happens after I stop to go forward or backwards, and it doesn't mater if I'm going straight or turning, and it only happens in first gear. It's bad when it happens. It's like you have a real real bad clutch. Shakes the Jeep violently. Then sometimes it won't do all day? That's why I thought it might be the limited slip diff? But the brake sticking would do the same thing? If I'm on a slight down hill and stop for a red light, and release the foot brake to go before I engage the clutch the Jeep will roll, so if the brakes were locking wouldn't I feel that then??
I would try lifting the rear and have someone press the brake pedal and hold it as if you're at a stop light, then release the pedal and turn the rear wheels and see if there's any binding. Also engage/disengage the parking brake a few times and look for the same thing.

If it's always rolling freely then I guess that isn't the problem. I was thinking maybe one side was sticking and binding up some causing the shuttering. Still wouldn't hurt to investigate it just to completely rule it out. You've seemed to have check just about everything else.
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Unread 07-29-2015, 09:04 AM   #14
glenn-91YJ
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Quote:
Originally Posted by Rwrangler View Post
it happens after I stop to go forward or backwards, and it doesn't mater if I'm going straight or turning, and it only happens in first gear.
The only time I had somewhat of a "shudder" was when I lifted the Jeep, installed 33" tires and was running the stock TC, DS and D35 (3.07). When starting in 1st I would get it...not real bad but it was there. I installed an AA SYE and TW drive shaft, shimmed the D35 and it's now smooth as silk!!!!

I know you haven't lifted it but it seems like it has to be the driveline or something with the rear end.
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Unread 07-29-2015, 10:06 AM   #15
ShadowSentry
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So this is one of those "broke my finger now my toe hurts" sorta things. Anyway, I have the same shudder only in first and REV. If you romp on the gas its there still but not so back. Anyway I had noticed that my idle had been sorta crappy when at a stop, so I ended up taking the throttle body off and cleaning the Idle air control and every inch of the throttle body. Put it back on the jeep and Reset the PCM/ECM whatever it is called and it had made a huge difference in how it idled when at a stop, smooth. Also I noticed that when starting off in 1st and rev the shudder was gone. it only lasted about 4 days, then it was back to crap idle. So I repeated the IAC clean and PCM reset and it fixed the idle and got rid of the shudder again for a few days. So my speculation is I need a new IAC and that may be contributing to my shudder issue or something to do with what the PCM is changing when reset. Just a really out there left field idea.
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