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Replacing very old rear diff oil, clean with sacrificial oil first?

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XC92

Proven Member
1,573
358
Jul 22, 2020
Queens, New_York
I need to replace the gear oil in my '92 Talon TSi AWD's rear diff as one of the final steps in a restoration project I've been working on since the summer. The car sat undriven for over 5 years, and before that probably didn't have the oil replaced for at least 5 years.

So I'm wondering if it makes sense to first replace the old oil with "sacrificial" gear oil, drive that for a while, and then put in fresh new oil, to clean things out.

I actually did this already with the trans and t-case when I dropped them a while back, replacing their old oil with Carquest Grease and Lube EP 90 GL-4 Gear Oil, which I intend to flush out and replace with Mitsubishi MTF 75W-85 for the trans and Redline 75W85 GL-4 gear oil for the t-case.

The "permanent" oil I'm going to put in the rear diff is Redline 75w-90 Synthetic Differential Gear Oil, which RTM recommends for my situation, a stock 1G DSM that it's going to be raced or driven too aggressive. I already have it, bought it a few months ago. It's GL-5, per FSM spec.

If using "sacrificial" oil this way is ok, can I also use the same EP 90 oil I put in the trans and t-case, even though it's GL-4 and the rear diff calls for GL-5, since we're only talking a few hundred miles, tops, and probably not even that much?

Or does it have to be GL-5, even for such a small number of miles? Or am I wasting my time doing this and should just put in the Redline GL-5 and be done with it?

Btw, either way, is a pump pretty much necessary to get new oil in the rear diff, or is there enough room to put a small funnel in the fill hole and slowly pour it in--or perhaps use a long flexible tube out the side or rear of the car (which should be fun when it's 35 degrees out)?
 
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Btw, either way, is a pump pretty much necessary to get new oil in the rear diff, or is there enough room to put a small funnel in the fill hole and slowly pour it in--or perhaps use a long flexible tube out the side or rear of the car (which should be fun when it's 35 degrees out)?

You will need a pump. $7 at Harbor Freight.

For the rest of your questions, I did this (unintentionally) with my transfer case last year. Swapped fluid (Redline whatever it is that's blue), drove it to work once, then got too paranoid about it and drained the fluid again. It was opaque black with chunks in it after 20-30 miles. If you have reason to be concerned, I'd say do the flush like you're suggesting. In terms of the fluid to use, I'm really unfamiliar with the fluids but from what I understand, the GL level dictates additives and proportions and are only safe on gears that were designed for them, e.g. if the manual calls for GL-5 don't use anything other than GL-5. But you'd be perfectly fine with a super cheap GL-5 for this quick flush.
 
Thanks. No way to this w/o a pump, then? $7 is cheap but this is something I'd only do every 5 years or so, so it seems silly to get. I was able to put oil in the trans and t-case with a funnel and tube. Obviously harder on the diff since there's no trunk opening for the tube to go through, but I thought a long and wide enough one might go around the side and slowly do the job.

Alternately, if I raise the rear enough, it seems like there's just enough room to do it entirely there, also with a tube and funnel. But I haven't been down there in a while so perhaps I'm being overly optimistic. Also, being raised, I assume that I'd slightly overfill it and would have to lower the rear with the fill open open to get the overfill out.

Btw did you figure out what caused those chunks coming out of your diff, and fix it?
 
I recently filled my rear diff with redline heavy shockproof- don't make the mistake I did! I filled up to the fill hole (with a pump) and then immediately went out on the highway and burned up the diff in 10 minutes! Turns out, the thicker oil sticks to the inside of the diff casing, and doesn't flow down quickly enough to properly lubricate the ring and pinion gears.

So the solution I was told was something called a "wet fill". Fill it, drive around the block (no faster than 30mph at the absolute most", then quickly jack it up and fill it before the oil runs back down to the bottom. - I don't relish the prospect of putting the car back up again, so when I do it I'm going to run it at 25-30 suspended in the air on jack stands before refilling it - but to each their own.
 
Sorry to hear about your diff. It's in the 30's here so that would make the oil even less viscous. I should probably wait till a warmer day, at least 50.
 
I think your change it twice method should be better than just fill and go. Perhaps see what comes out when you drain it each time. If it’s concerning, then take the cover off for a better inspection. If its ok, then just do the second fill and call it good. I filled mine while the gas tank was out. I’d buy the pump if I were in your shoes, since you already plan to fill it twice. $7 to $10 is worth it.
 
It's less about saving money than preferring to "hack" solutions to rarely encountered problems. Like, everyone needs a ratchet wrench for auto work and it's hard to imagine doing it without one, probably several. But if you replace seals only every 5-10 years then you can probably get by with a screwdriver and a stead hand instead of buying a seal removal tool.

But you're right, it's only $7 or so, even less with those 25% off Advance Auto coupons. And I'll probably find other uses for it, like removing excess oil accidentally added to the engine.

Btw for this "sacrificial" oil, does it matter if it's synth or conventional, and should it be formulated for a diff with LSD, i.e. with friction modifier additives?
 
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BTW for this "sacrificial" oil, does it matter if it's synth or conventional, and should it be formulated for a diff with LSD, i.e. with friction modifier additives?

No to both.

The LSD units used in DSM's are a sealed viscous coupler so there isn't any need for friction modifier additives.
 
Thanks! This is the sort of thing the FSM doesn't address, at least as far as I recall. And that's what I was thinking, what does it matter if there's a LSD if it's sealed.

So basically, any decent conventional GL-5 75W-90 or 80W-90 should work, if I only intend to keep it in there for a few hundred miles tops? Should I add any additives to help it "clean" the diff, and if so what, or should it be ok as is?

Btw I intend to put Red Line 75W90 GL-5 after I dump the sacrificial oil, per RTM Racing's rec. I'm not running anything special, everything stock, so I assume I don't need Shockproof, right?
 
It's less about saving money than preferring to "hack" solutions to rarely encountered problems. Like, everyone needs a ratchet wrench for auto work and it's hard to imagine doing it without one, probably several. But if you replace seals only every 5-10 years then you can probably get by with a screwdriver and a stead hand instead of buying a seal removal tool.

But you're right, it's only $7 or so, even less with those 25% off Advance Auto coupons. And I'll probably find other uses for it, like removing excess oil accidentally added to the engine.

Btw for this "sacrificial" oil, does it matter if it's synth or conventional, and should it be formulated for a diff with LSD, i.e. with friction modifier additives?
I use clear Aquarium Tubing on the end of the bottles with the nozzle tips to fill mine up (well, until I got a liquid transfer pump, and still use clear tubing). The tubing allows you to see the fluid going into the unit you are working on.
 
Sorry to ask again, but for the purposes described, cleaning out the old rear diff oil with cheap "sacrificial" oil that'll be dumped after a few hundred miles at most, and without driving the car too aggressively or putting the rear diff under major stress (which I have to do anyway as I'm also breaking in a new clutch), is it ok to use GL-4 EP90 oil, which doesn't have the phosphorous and sulfur additives that GL-5 has, or should I stick to GL-5?

It's just that I happen to have just enough of the GL-4 EP90 left over to refill the rear diff and was hoping to use that up and save a few bucks. But if I need GL-5, that's what I'll get and put in.

Btw, when the rear is up and I turn one wheel by hand, the other wheel turns the other way. Does that mean that the rear diff's LSD VC is shot and needs a new one? I've read that on 1G DSMs they tend to go after around 70k miles, and mine has around 78k.

Same for the front?
 
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