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This is my first time to post in an auto forum. It has been a few years since I have done any mechanic work, but the need dictates for it now.


I have a 2001 Silverado Z71 with 143,000 miles. Last week I noticed a little "kick" while I was driving and then a small vibration under my feet. It wasn't a big thing, but enough out of the ordinary to warrant me pulling over. When I checked under the vehicle the underside of the chassis was coated in oil above the front driveshaft U-joint to the transfercase flange. The transfer case itself was covered in old grime and somewhat fresh oil. I drove the short few miles home without any vibrations or noticable change in the drive of the truck.


I put all 4 corners of the truck up on jackstands and proceeded to try and figure out where the oil was coming from. The transmission vent hose was wet with Dexron and a clear shot from the hose to the underside was grimed oil, too dark to tell what color. The transfer case looked as if it had been the victim of a slow drip from the vent hose, but once again, too dark and grimy to tell what color. The oil above the U-joint and splash did have a green tint. So I could guess that would be gear oil. Yet where is it coming from. I have wiped everything down completely clean; transfer case, U-joint, and undercarraige.


With the wife on the accelerator in 2WD at 1500-2000 RPMs there are no leaks. I am assuming because there is no "road pressure" being applied to the differential and what not. There is oil in the transfer case. I cannot tell because I have never played with a U-joint before, but I believe the yoke is worn. I think this because when I grab the joint there is slight movement, bare movement up and down in the yoke. I cannot twist the U-joint itself though. The inside of the U-joint was covered in oil when I first discovered the issue. Oh, and the transfer case was only covered in oil on the vent hose side. The passenger side was dry and clean. The rock shield and underside of the transfer case was wet, but once again, I believe this may be because the u-joint was slinging it. I may be very, very wrong though.


When my wife applies power and the driveshaft is spinning I hear a definent and obvious clunk every few seconds. At first I thought it was the driveshaft itself. But as I followed the driveshaft to the rear of the vehicle it was getting louder. It is either coming from the G80 differential (checked in the glovebox to make sure it was a G80) or the back of the driveshaft itself. Also, when this "clunk" happens the left rear wheel engages and spins.


So, I have three questions:


1. Does the oil/yoke sound like a familiar common issue?


2. With the G80 rear differential off of the ground is this the way it is supposed to work and the "clunk" normal?


3. If the G80 is faulty could it be pulling on the driveshaft/ yoke or is the yoke just a wear and tear item?



Any help or guidance would be greatly appreciated. Thank you for reading my post.

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Def seems to be a leak somewhere, I would check and fill the transfer case and tranny to see if any are low both when cold.and.hot. the g80 also seems to.be doing its job looking for traction so.I.wouldn't worry about that.

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Search for transfer case pinhole leak on this site for pictures. There is a long thread about it. However, your leak sounds faster.


The clunk at the rear is your G80 engaging.

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  • 2 weeks later...

Thanks for both of the quick responses gentlemen. Unfortunatley, my response was delayed by a quick trip to the VA hospital. However, before I left I had drained and added new Auto Track II to the transfer case and lowered the rear end off of the jack stands for the night. I was going to lower the front the next morning and go for a test ride to see what splashed up, but that is when I got derailed with the VA stuff. I had cleaned the case, yoke, u-joint, and under carraige so well that I used Q-tips to wipe out the u-joint. Before leaving to go see the doctor I put paper towels underneath to see if I could catch anything.


Yep! Right underneath the U-joint the brand new bright blue Auto Trak II was dripping from the inside of the yoke. Q-tips on the face of the yoke confirmed it for me. I don't have the weep hole yoke, so I am trying to read up on how these yokes are pieced in to the case and what seals I need to replace or if I need to replace the entire yoke and seals. I am going to go ahead and do the u-joint while I am at it since they are fairly inexpensive. Not sure about the yoke though?


Once again, I appreciate the advice. I wasn't being rude. I was just detained for a bit.

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  • 3 weeks later...

It ended up being the rear seal of the transfer case where the yoke slides in. As the sliding yoke was doing it's job (sliding back and forth in the transfer case) the pressure in the transfer case was blowing it out of the seal. So when the yoke would slide out it would toss the Auto Trak to the rear. When it would slide back it was tossing the fluid back onto the transfer case itself. There was a definitive line of Auto Track II across the top of the underside right above the yoke side of the u-joint from one side of the truck to the othe other.


New seal was about $90 from the dealer. Used an oil filter socket to tap it in. No leaks since install.


I've got the kit on order for the pump. Since I've had this problem I've read too much about pump rub to not do anything about it now. Thanks for all the advice gentlemen.


Have a good New Year.

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