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I would like to change all the fluids in my recently acquired 81 C3 with 108,000 miles on it. Auto trans fluid already changed, but now ready to change the differential fluid. Lacking a drain plug, I assume I have to use a suction gun to suck it all out. My question is, can I get the hose in the filler plug hole okay, and will it get anywhere near the bottom? The other question is, I have a very slight gear whine at 40mph. Can I up the viscosity from 80/90 to 95/140, and will it soften(?) the whine? The addition of the posi fluid is noted if the selected gear lube does not have it in it already.
Yes, a suction hose will fit in the fill hole. It does however take some time. Just be patient and get out all that you can.
Refill until it starts flowing out of the fill hole. I followed the advice of the Corvette Forum, Tech Coordinator, Gary Ramadie, and used Lucas 85w/140 with the GM additive. https://www.corvetteforum.com/forums...post1598005034
I just changed the diff oil on my 71 and I used my harbor freight pneumatic brake bleeder to suck out the oil. it didn't take long and got all but a couple oz. out of it. x2 on the lucas 85-140 plus 6 oz of gm posi oil.
If all else, I believe the plug will take either a 5/8 or 3/4 open-end wrench. And put a little thread sealant on when you re-install the plug.
Some people think you need to do figure 8s to mix the two fluids. That was old school and a good idea with other cars that have hollow axle tubes. The differential on a C3 is so tight, there is no place for the fluid to go but around the gears anyway. Its mixed before you pull out of the driveway.
I pour a unequal amounts of the two fluids into a clear squeeze bottle with a spout. (Those can be found at a food industry store) Then shake the bottle a few times and pour into the diff hole.
Last edited by HeadsU.P.; Jul 3, 2019 at 03:52 PM.
Your most difficult task will be to remove the drain plug (depending on how long it has been since last removal). I had to remove the rear spare tub and put a 4 foot breaker bar on a good socket to get it started. A big air impact gun wouldn't budge it....
I did mine slightly different, its more like a drain, flush, drain, fill.
1. Drive until hot. Drain. (you'll never get all the fluid out)
2. Refill with generic cheap fluid.
3. Drive until hot. Drain (after this second drain you'll be getting out a lot of the old fluid that mixed with the new)
4. Refill with good high quality oil containing friction modifier.
I did mine slightly different, its more like a drain, flush, drain, fill.
1. Drive until hot. Drain. (you'll never get all the fluid out)
2. Refill with generic cheap fluid.
3. Drive until hot. Drain (after this second drain you'll be getting out a lot of the old fluid that mixed with the new)
4. Refill with good high quality oil containing friction modifier.
If you have the time and patience, excellent idea!
Hopefully not hijacking here but i get a skipping or stutter from my rear end on my '71 ... mostly turning corners or when accelerating from a dead stop after the car has been driven for a while. Would changing the differential fluid as described above be a good starting point to resolve this, or do i likely have bigger issues?
Hopefully not hijacking here but i get a skipping or stutter from my rear end on my '71 ... mostly turning corners or when accelerating from a dead stop after the car has been driven for a while. Would changing the differential fluid as described above be a good starting point to resolve this, or do i likely have bigger issues?
Changing fluid with two bottles of the GM additive will probably cure your problem.
Thanks for all the info and suggestions guys. And yes I have a set of 8 point sockets. It's the only way to handle pipe plugs.
Not to discourage changing the fluid but a buddy of mine has an 81 with the same gear whine you have. He went through the fluid change using the higher viscosity fluid and additive and it had minimal effect on the whine.
Differential Fluid Change
