I need some help here. I have an 87 Formula 350 and I am having an on and off again weird problem coming from the rear of the car. Not sure if it's the rear or what it is, but sometimes, while taking turns or per say driving through a parking lot swaying/steering back and forth, I have a terrible vibration and a metalic sound of some sort. It is not a constant problem, but it has occured twice in the lasw two weeks and it sounds like something is gonna come apart back there. ANy idea what this is.? Is my posi going? It is the wierding thing and like I said, sometimes it there, sometimes not, but it's not a good feeling when it's there. Can this be U Joints or a bad tranny mount. I have a bad feeling the rear is going or my posi, how would I be able to tell? Any help would be greatly appreciated

 

Remove the rear cover, drain the oil, inspect and refill, be sure to add GM friction modifier.
When is the last time the rear has been serviced and what brand friction modifier was used?
It sounds like it could be chatter from the posi. My auburn did that bad usually only when hot, even with 16 ounces of cheap posi additive, I changed the gear oil with GM gear oil and 4 ounces of friction modifier additive and it solved the problem.
Posi rears need to be serviced more often than conventional rears.

 

Last summer I noticed a rumbling sound coming from the rear of my IROC...it was only when I took sharp right turns and only on dry pavement....somedays it would be there and others it would be gone!

I asked this question on IROCZONE's message board and one of the guys (TtopIroc) wrote the following....

" ok im curently going to technical school and this is a problem for all posis. This is what my teacher told me to do, just add some posi fluid to the rearend, thats it!!! I promise this will solve the problem ive done to alot of cars and its always worked.

NOW ILL EXPLAIN WHY IT DOES THIS:

as your turning with a posi trac both wheels are trying to turn at the same time and the rearend gears will slip causing this harendas noise that makes u want to **** in your pants. but the fluid is specially designed for positraction rearends and will lubricate the gears corectly. I think the will sell it as an additive in your local parts store.

THE NOISE WILL COME BACK BUT ADDING THE FLUID WILL SOLVE IT EVERY TIME "


I never had a chance to do this as I put my car away for the winter shortly afterwards... but this spring I will be doing it and I am pretty sure it will solve my problem!

Oh and the Posi additive is like $3 or $4 at any GM Dealer.

Hope this helps.

 

Thank you both very much. I was hoping this would be what it was and it's a big relief. Cannot thank you enough

 

I thought you were not supposed to use the posi additive to the BW 9 bolt rear ends, but I may be wrong.

 

What would you think that?

 

Because GM said not to in a service bulletin.

 

If this is a 9-bolt (which it HAS to be for an L98), you may want to read these threads:

https://www.thirdgen.org/techbb2/sho...threadid=72062

https://www.thirdgen.org/techbb2/sho...threadid=77011

What you do is your choice. I plan to do what is stated in the second thread - I just haven't had time to get to it yet.

 

So, what the heck should I do? I don't want to burn it up, but what do I add? I am gonna call a GM service center and see what they say, but I don't know if I should trust them or not.

 

As a previous thread states, the GM parts manager that I know said that the additive is only to reduce noise. If you believe this and the noise is driving you nuts, you probably should put the additive in. However, you may want to consider putting in a good, synthetic oil (like the Red Line that I plan to use) and see if the noise persists. If it does, put the additive in. If it doesn't, leave it alone.

This is how I plan to approach this.