# Need help in determing what the dealership put in for diff fluid........



## pctek (Jul 18, 2010)

I wanted to see if anyone can figure what the dealer put in late last year when my used gm certified warranty was still good shortly after buying my goat. Here I'll paste what I stated to the service writer when I took the goat in and then paste what jobs they did and such. 

Before pasting info let me tell you briefly that I still have clunking and whinning noise from the drivetrain area, but the clunking was worst back when I first bought the car before the dealer did the fluid exchange. The whining noise never did go away still happens from the 40mph to 50mph or so spot. And the clunking even after they did the fluid change was still present but not as severe, seems lately its getting worst.

I know maybe the clunking can be a bad loose driveshaft carrier bearing, but before I check that out I was thinking I can do the easy diff fluid change with thicker diff oil becuase what I think happen is that the dealer put in that infamous thin 75/90 weight. Thanks...

"Customer states there is a whining noise from rear of vehicle.
Most noticable between 40-50mph
We did find noise from rear diff.
removed diff cover to inspect for bearing/metal material, none found.
Refilled with updated fluid & very little noise heard."

Qty: Number Description

1 #88862624 Fluid 8.800

2 #89021809 Oil 8.800

PO#87550 Stow
2 Price-Diff

^^^^^ I dont know what this Means.


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## HP11 (Apr 11, 2009)

88862624 - DESCRIPTION: FLUID 32 FLUID OUNCE, BOTTLE, DEXRON LS GEAR OIL 75W-90 AND CONTAINS LIMITED SLIP FRICTION MODIFIER OFFERED AS A PACKAGE QUANTITY OF 1. 

89021809 - DESCRIPTION: OIL (That's all tha came up on that one)

I thnk those are both GM branded/bottled items. The lower number is just synthetic 75w-140 GL-5 and the above number is 75W-90 that already has friction modifier mixed in. The rest of the items are the purchase order number and the name of the technician. Don't know what 2 price-diff means. 

It's mentioned in this thread: http://www.gtoforum.com/f3/gm-diff-fluid-34953/


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## pctek (Jul 18, 2010)

So when putting in friction modifier should I start with the lowest amount first 1.5 oz? Because its easier to add more than drain the whole thing again right?

So if theres not enough FM how can you tell, what symptoms occur when theres not enough?

Also as long as the fliuds is gl-5 requirement and correct weight and such can you also add any FM brand or does it have to be the torco type f one? Thanks


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## HP11 (Apr 11, 2009)

The manual says 1 oz. of FM. Don't know what the symptoms would be if you didn't have enough but I suspect it would cause premature wear in the lsd unit. You should be able to use any GL-5 synthetic and any FM that is equivalent to Gemeral Motors 7098. Based on your description of the problem, however, I don't see how changing it again will solve your problem.


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## pctek (Jul 18, 2010)

HP11 said:


> The manual says 1 oz. of FM. Don't know what the symptoms would be if you didn't have enough but I suspect it would cause premature wear in the lsd unit. You should be able to use any GL-5 synthetic and any FM that is equivalent to Gemeral Motors 7098. Based on your description of the problem, however, I don't see how changing it again will solve your problem.


The General Motors 7098 bottle where or how do you can tell that the bottle is 7098? What is that number anyways? I thought the part # for those bottle start with 89........ the long # like the gm gear oil?

I think my local Chevy dealer might have some bottle of fm in stock, so how can I tell it's the FM for our gto diffs and not another FM for other gm cars?

Well I'm thinking of changing it cause I'm not confident they put the right amount of FM or who knows what they put in, I think they put the thinner 75/90 gear oil that's suppose to be for corvettes and other cars. I heard gm change the part# to supersede to that new 75/90 # and I thought the gto diffs are different type of lsd diff than other gm cars. So I'm wondering if the thicker oil will quite down the clunking and also take away that whine noise.

When I first bought the car the clunking was really bad, but when they put in new diff oil it was better but still slight there, now it's got little worst lately, so again if they put thinner gear oil and the combination in not putting in the right amount of FM, maybe that's why I'm getting some noise's. Thanks


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## HP11 (Apr 11, 2009)

Remember, the noise was there before you had the oil changed so a problem already existed. It quieted down when they changed the oil. Fresh/new oil may do that, but it won't 'fix' whatever is wrong. It's noiseier now because mechanical problems tend to get worse over time; not better. The first quart of oil listed on your invoice already has the FM in it. The second quart is the 'heavier' oil. A 50/50 (or thereabouts) mix of the two really wouldn't be signifigantly thinner than 75W-140, not enought to matter. That number (7098) is the description as listed the manual, as in 'Limited-Slip Differential Friction Modifier 7098' . The GM part number is 89021958.


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## jpalamar (Jul 22, 2008)

I ran into this at my local dealer years ago. I went in to pick up some fluid and the guy handed me 75w90 and I was like dude, my manual says 75-140... he said it was the new part number. Needless to say, I waiting and got fluid somewhere else that was 75-140 w/ FM in it.

Other option is to buy some 75-140 and and the propoer amout of FM to the mix.


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## HP11 (Apr 11, 2009)

I could go into a study on fluid flow rates but I ain't opening up that 'can of worms' here. I'll just say that the flow rates when cold are different between the 75W-90 and 75W-140 gear oils because the 75W-140 takes longer to flow. That is why GM dealerships are using 75W-90 gear now. Quicker flow, quicker lubrication when cold. They're pretty much the same when hot as heat will only thin gear lube to a certain degree and, at that point, the differences in thickness would be minimal. Bottom line, however, like I said, neither wt. will 'fix' a noisy differential.


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## pctek (Jul 18, 2010)

I just put in mobile 1 75/140 syn gear oil n motorcraft xl3 FM and the clunking got worst. What was in there before was gm fluids late last year when the clunking was even the worst n the used gm certified warranty covered the oil change the dealership did. They probably put in that new 74/90 im assuming.

So I guess I'll give it one more shot n try torco rgo 85/140 that eveyone raves about. thanks


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## Falco21 (May 11, 2010)

^^ Don't forget the Type F Friction Modifier


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## pctek (Jul 18, 2010)

Falco21 said:


> ^^ Don't forget the Type F Friction Modifier



Right I did order the FM bottle aswell along with 2 quarts of the RGO 85/140.

About how many ounces of FM do I put in? Start off with 1 oz like gm says or what thanks?


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## jpalamar (Jul 22, 2008)

pctek said:


> I just put in mobile 1 75/140 syn gear oil n motorcraft xl3 FM and the clunking got worst. What was in there before was gm fluids late last year when the clunking was even the worst n the used gm certified warranty covered the oil change the dealership did. They probably put in that new 74/90 im assuming.
> 
> So I guess I'll give it one more shot n try torco rgo 85/140 that eveyone raves about. thanks


Mobil 1 gear oil already has FM in it...
Mobil 1 Synthetic Gear Lube LS 75W-140


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## Falco21 (May 11, 2010)

pctek said:


> Right I did order the FM bottle aswell along with 2 quarts of the RGO 85/140.
> 
> About how many ounces of FM do I put in? Start off with 1 oz like gm says or what thanks?


1.5-2 oz. 

Mix it in with the first quart of oil.


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## pctek (Jul 18, 2010)

Really interesting yesterday as I said couple post's before here, that after putting in mobile 1 75/140 and about 1oz of ford FM I went for a test drive and it was clunking worst than what I had in there before which was a mix of the new gm fluid 75/90 and the old OG GM 75/140 according to the work order sheet from dealer when it was change out late last year sometime.

Well today for the heck of it for the meantime before the tocro rgo 85/140 arrives, I added the rest of the bottle ford fm. Well then I took it for another drive and the clunking is not bad no more, pretty much the way it was before or maybe tad better, just once n a great while when driving.

So im not sure if adding FM it helped with the clunking or the mobile gear oil worked in through the diff since the last drive yeasterday. If it was the FM I thought FM was only for chatter for the clutches not to bind to each other so much? Thanks


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## Falco21 (May 11, 2010)

FM can definitely help out the noise and clunking from the rear. A lot of people recommend adding more FM if the clunking is still occurring. Either way, I would change that fluid out and see what happens. Even if it does not get any better, as long as the RP is out of there.


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## pctek (Jul 18, 2010)

Falco21 said:


> FM can definitely help out the noise and clunking from the rear. A lot of people recommend adding more FM if the clunking is still occurring. Either way, I would change that fluid out and see what happens. Even if it does not get any better, as long as the RP is out of there.


Its not RP in there its mobile 1, but yea I will still change it out to torco see how that works.

If im right the regular gear oil is more thicker/heavier than say synthenic, whats got me trippin/worrying, is make the mpg drop some because of more drag resistance?


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