My manual tran. YJ Wrangler shakes under power, above 2k RPM... thanks - Jeep Wrangler Forum

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Old 06-13-2014, 05:22 PM   #1
Join Date: May 2011
Location: Merritt Island, FL
Posts: 31
My manual tran. YJ Wrangler shakes under power, above 2k RPM... thanks

Hi everybody, thanks for looking and commenting.
I have a 1991 Wrangler 4.0 5spd that's pretty much stock. I've rebuilt a lot of its mechanicals and until this week it ran perfect. Three days ago the Jeep developed an infrequent noise that sounded like it was coming from the muffler. I crawled underneath and noticed what looks like the exhaust pipe to the muffler's front connection had worn loose and developed play in the connection. The following day the metal on metal chirping noise grew louder and more frequent. Not being a pay week I made the mistake of JB Welding the connection to quiet the Jeep, until payday. In case you're unaware JB Weld burns at high temperature. For my whole commute to work it gave off a very strong chemical smell. I rolled down the windows and knew it wouldn't be long before I could do a real repair.

BUT then yesterday I noticed THE REAL PROBLEM.
My commute is mostly highway miles where I drive around 55 mph. The Jeep developed a pronounced shake above 2k RPM, in all gears. As RPM increases so does the severity of the shake/vibration. While driving home I began to problem solve and these are some of the clues I hope you guys can use to help me solve my mystery.

When I'm driving above 2k rpm the Jeep shakes. If I push the clutch in the shake disappears. With the clutch in I rev the engine and the engine sounds normal and the Jeep doesn't shake. That pretty much illuminates the suspension, steering, or engine as the problem. I'm assuming the problem is something to do with the manual transmission. On a side note I've owned the Jeep for about 4 years and ever since I've had it, getting it into reverse has been a slight hassle. Sometimes it slips right in, and other times I have to give it a small amount of RPM and slightly let out the clutch to allow it to slip into gear. This only happens with reverse. Given the transmissions prior performance I just learned to live with this little quirk.

I've had automatic transmission cars my entire life so I'm kind of unfamiliar with the manual transmission found in the Jeep. I noticed there is one (loose) bolt fastened the transmission to the skid plate. Is this bolt supposed to be snug? ...or does it allow for transmission movement during shifting?

Any ideas or tests I could perform to help isolate the problem would be GREATLY APPRECIATED. I'm thinking about trying to replicate the problem while the Jeep is up on jackstands. My friend will slowly operate the Jeep while I crawl underneath to inspect.

Thanks again everybody.

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Old 06-13-2014, 05:36 PM   #2
Pmryall's Avatar
Join Date: May 2014
Location: NW Ohio
Posts: 2,374
It should be snug, I agree it sounds like your transmission, enough fluid, maybe drain it and look for shavings. I had some shifting problems ahwhile back that went away when I changed over to synthetic, sorry I'm not a transmission guy so those are my humble thoughts.

Keep on jeepin' on.
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Old 06-13-2014, 07:17 PM   #3
c5wagner's Avatar
Join Date: Apr 2013
Location: Reno, Nevada
Posts: 1,480
Check your U-joints first, if those are fine, put your transfer case into neutral and try going into gear and get your engine up to 2000 rpms. since the transfer case should be in neutral you won't go anywhere. If the vibration isn't with the tranny, then the transfer case could be the problem.

The shake could also be a warped flywheel or your pressure plate/ clutch disc can be unbalanced .
1989 YJ, 4.2l, Junkyard TBI with timing control, aw4, np231 JB conversions super short SYE, D30 ARB & Ford 8.8 ARB 4.56, 35x12.5r15 Goodyear MTR's, bds 3.5" springs, 1" body lift, and 1/2" Boom Shackles.
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Old 06-13-2014, 07:44 PM   #4
173ABN VN's Avatar
Join Date: Jan 2013
Location: rural N.E. Kansas
Posts: 266
ditto on what C5wanger said,I'd check the U-joints...russ
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Old 08-08-2014, 09:52 AM   #5
Join Date: May 2011
Location: Merritt Island, FL
Posts: 31
Thanks Pmryall, c5wagner, and 173ABN VN for your help.
For anyone else in the future that will have this same problem. Here's the update.

I didn't notice anything wrong with with the U-Joints so I was forced to have my local (honest) mechanic take a look at it. Initially he thought it was the yokes, but he spoke with a 4x4 expert and he said that wasn't my problem. My problem was related to the U-Joints!

While my visual inspection didn't reveal anything obvious, the 4x4 expert suggest my mechanic remove the drive-shafts for a thorough inspection of the U-Joints. The culprit was the pinion yoke straps that bolt to the yokes. Over time (a very small amount) material wears away (called galling) and light actually shined through exposing a small gap.

New Spicer U-joints were installed and the pinion yoke straps had to be replaced. These were sourced from the local Dodge Dealer.

While I could have done this job myself, I was really at a loss as to what the actual problem was. I needed the mechanic's expertise to confirm the source of the problem. So in the end it cost me $325. for parts and labor. If the price of the repairs seems high, it was because he made other repairs to my jeep. I listed the price to illustrate that the cost wasn't bad.

I'm including a link/pic to illustrate what was the cause of the problem.

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1991 , shake , transmission

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