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Department of Redundancy Department
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Discussion Starter #1
Here's the deal.
'01 Sonoma. Ext cab. 4.3 auto. 3.42 w/locking rear end. ZQ8, if that matters.
Truck has less than 6000 miles on it.
Changed the engine oil at 5400. Manual says to change rear end lube when engine oil is changed. Haven't done it *yet*, but plan to this week.

Did a 250 mile road trip yesterday. As I was nearing the end, I noticed that when I turned left from a dead stop, I'd feel a rumble or shudder until I got up to 5-10 mph or so. Doesn't do it on right turns. Also didn't do it this morning, when the truck was cold.

Is my POSI rearend shot? Will changing the gear oil fix the rumble? Do they come with special "break in" lube, that has different characteristics than the 80W90, or what?
Also, I was surprised that my shop manual specified the same gear lube (80W90) for the posi as for the "open" rear end.
 

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My owners manual says the locking rear diff fluid is supposed to be changed at 1000 miles, and than every 15,000 after that. Like Joe said, let the dealership warranty it for ya, and the friction additive is in all 80W90 gear lube.
 

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Department of Redundancy Department
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114 Posts
Discussion Starter #4
Every 1000 miles?! Yikes!
My owners manual said to change with the first engine oil change.

Drove it for a while tonight. Hasn't acted up.
If it does it again after I change the lube, I'll definitely take it to the dealer. Even if they can't find anything, at least it'll be documented. (Lemon laws are a good thing.) :D

Thanks for your responses.
 

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I'd Rather be Blown than Injected
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251 Posts
Your clutch pack in the posi is sticking. Change the oil and add the
GM additive. Or go to a Syn oil with the additive in it. Then warm up the rear and do some figure 8s to get the oil into the clutchs.
 

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Department of Redundancy Department
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Discussion Starter #6 (Edited)
Since I found conflicting info in the owner's manual and the shop manual, I went to the dealer and talked with one of the techs.

Here's the story... The 2001 (not sure what other years) Sonoma uses an Eaton locking rearend. Not the normal "clutch pack", as I understand it. He said what I described is *not* normal, but he wasn't able to get it to do it when we test drove it.
In any event, he said that the first thing he would do would be change the lube. Gonna do that tonight. Have another long road trip tomorrow, so we'll see.
BTW., per the tech's notes... (type-written, in a document protecter in a ring binder, *not* the shop manual) the 2001 Sonoma uses the same gear lube as the "standard" rear end. Essentially 80W90. He said it was the same stuff that I could buy at a parts store. *No friction modifiers are to be added.* (This backs up what the owner's manual said, BTW.)

Also found out some other info... The ZQ8 and X-Treme trucks have a common affliction that causes them to vibrate when accelerating from a standstill. They think it has to do with the lowered suspension and pinion angles, but strangely enough, nobody has found a fix for it yet. He pointed it out in my truck. I'd noticed it, but didn't think much about it. He also said that the 4 cyl ZQ8s were worse. (Maybe because you can get them with the 4.10 axle??)

Anyway, the service tech was most helpful. I already buy my parts there (doing a Fiero engine swap). Thinking about sending them some repair business, if the need arises. (I didn't buy my truck there.)
If your in West Atlanta, check out Lou Sobh Buick-Olds-Pontiac.
 

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I'd Rather be Blown than Injected
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251 Posts
Going to have to pull out my shop books and check this out.
I have put a 4.10 in my truck and have no such noise or vibration
Maybe he means the trucks that don't have the no hop shock?
Yea they get wheel hop when you jump on it.
 

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hey guys, i have a 01 Blazer Xtreme, my rearend shutters on left turns also, took it to dealership and before i even finished describing the problem the mechanic said it should do that, and you may also hear clicking noises. he said as turn going from 0-10mph the rearend senses slip and locks, if you notice it as you come to a stop it may be unlocking, i wouldn't worry but after talking to use guys about the 1,00 miles rule, i think i will change mine this weekend, i have about 10,000 miles on mine, thanks guys!!!!
 

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Department of Redundancy Department
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Discussion Starter #10
I have two vibrations. One is from a dead stop, going in a straight line. It's noticeable at half throttle. It's *not* a hop. Just a light pulsation. All done by 10 mph. If I stomp it in wet weather, it hops like a MoFo. Not really enough power to break 'em loose in the dry, though, unless I'm going around a corner.
The other vibration is the rear end chatter. I drove 80 MPH for 175 miles today. Twice. After getting hot, it chatters in right and left turns, from a standstill.
Trying to remember if I've ever felt it thump (unlock?) when I stopped. I'll have to pay attention.

I *didn't* have time to change the lube last night, like I was going to. I'll let you know if it fixes the symptom, when I do.
 

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the vibration straight off the line is normal, although gm has no explanation why all ZQ8 and Z87 trucks do it, my truck does the exact same thing you're talking about after a long ride during left and sometimes right turns, i'm going to change my lube this weekend too, let me know how yours goes afterwards.
 

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I'd Rather be Blown than Injected
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Don't have none of that stuff going on in my truck.
 

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Department of Redundancy Department
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Discussion Starter #13
Wicked X said:
Don't have none of that stuff going on in my truck.
It's Broken! Send it back! :D

J/K...
Guess you got one of the ones that all of the production tolerances stacked up in your favor. It happens. Rarely, but it happens.
 

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Cali Member
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I've also experienced that problem with my 01 Blazer Xtreme

it was an acceleration shudder only when turning LEFT from a DEAD STOP. The dealer techs installed 4 degree shims due to the fact that they concluded it was pinion angle. It helped, but didn't wipe it out completely... and the guy even said the shudder is a lot lighter, and not as consistent as before, but it is a common problem (in other words saying its NORMAL, so deal with it.) :thumbdown

ANYWAY, I got my 3" blocks in there now with 2 degree taper, and I haven't heard it since. Very rarely, and even when it does act up it's really faint.
 

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Primer Spots
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3,713 Posts
Also found out some other info... The ZQ8 and X-Treme trucks have a common affliction that causes them to vibrate when accelerating from a standstill. They think it has to do with the lowered suspension and pinion angles, but strangely enough, nobody has found a fix for it yet.

I took mine to 2 dealers about the same problem, they said it was normal? WTF? maybe pinion shims would work?
 

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Same straight line rear-end vibration in my Sonoma ZQ8 with 400 miles (not a typo...four hundred). "Suburbanitus" is what I've heard this pinion angle thing called before.
It's in the same shop that Raydar has mentioned. After a "bad tire" was replaced at 388 miles, the suspension service tech was straightforward about the situation.
Since there was no discernible improvement, I returned this AM. I suggested indexing the driveshaft and "...maybe pinion shims or something." when I left it. I told them that IF they decide it can't be remedied, to put it in writing so I can begin "the process". (GA lemon law) No word yet. I'm driving a dealer-paid rental for now. Wish me luck!

N. J.
 

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PCMPOWERTUNES
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I have figured it out

I have a 02 X with 496 miles on it and it had the shudder from day one. I thought it was tire balance . I checked today on the alignment rack with the rear wheels up in the air and put it into gear and guess what I found:finger: :finger: :finger: :finger: :finger:
The ****ing driveshat is bent. So for everyone else out there with this problem check it out I can guarantee thats what your shudder is.
Jim
 

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2 Litre Terror
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Thank God. I thought I was going crazy. I had my rear replaced under warranty (4200 Miles ) also had them swap out the 3.42's for 3.73's. I still have a slight shudder when accelerating aropund a turn. Especially noticeable if I take off slowly. The dealer said they couldnt duplicate the problem. I am calling them tom. to see about getting the truck back in and having the shims installed.
 
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