Rear end noise...No, not that rear end

The background:

The car is an 83 RX-7 GSL with the LSD. It has over 180000 miles on it. I decided since I haven't ever changed the rear end lube that it would be a good idea to do so. I also decided to use synthetic for the first time in anything. The lube that came out was gross, it might have come from Japan with the car. There were two pieces of metal stuck to the plug magnet. I would call them slivers, not large, but not the usual fuzz either. Before the oil change, the axle was pretty quiet. It had a very slight whine when coasting, but other than that sounded to be in good shape. Now it has a noticeable whine when power on slightly(cruise) and coast, and a clunk when transitioning from coast to power on.

Lube used was Mobile One 75w-90 with a LSD additive.

The questions:

Is it normal for synthetic gear lubes to be noisier?

Since it is a high mileage axle should I have stayed with the smelly stuff?

Should I go back to the smelly stuff?

Your thoughts and guidance are most welcome.

Toyman01 wrote: Is it normal for synthetic gear lubes to be noisier?

It shouldn't be. I replaced the rear diff and tranny fluid in my Impreza recently with Red Line 75W-90 (friction modifier in diff, no friction modifier in tranny). There's been no change in the noise level from either. LSD fluid in my FC was changed before I bought it, and there's no noise coming from that, either.

there is a good chance because you took out all the grit that was left from years of abuse that it now nothing to absorb the play in the gears. that said maybe it is just the brand. try a higher end brand such as redline or Amsoil for your rear end Lube.

the clunk could just be the u joint that had not showed up before.

The diff in Stinky whined with Red Line in it, it was more noticeable because of no carpet etc. I wouldn't be overly concerned with that.

The R&P backlash is .003 to .005 IIRC (my manual's at the house). This equates to ~1/4" of rotation as measured at the pinion flange. If your pinion has this measurement and most important can't be wiggled up/down or moved in/out (even a tiny bit!), then I would keep on gettin' it. If you CAN move it, then it's time to investigate.

The clunk can come from several places like the aforementioned U joints, also if the limited slip thrust washers are worn then you will feel the diff 'clunk' on lockup as the spider gear pins force the races apart and the plates lock.

Hears what i would do. Drain and refill with ATF drive 5 - 10 miles and drain and refill with LUCUS 90-140w. Go have fun.

I don't know whats in that stuff but it works. My dwarf dif temp droped 60 deg F. I use to burn my hand checking the temp now it warm but not hot.

The ATF will help clean out the goo and clear the posi plates of crud.

44

+1 on ATF...cleans anything...also very useful as an engine flush agent without the damage the commercial flushes can have on old seals and gaskets. I used to work at a quick lube place that had pretty good training (in class type and with fairly thick textbooks full of good stuff...if only H.S. were as entertaining...sigh) and I learned that ATF - dexron especially-has a lot of oil based detergents that clean without acting as a solvent.

Machinist's Workshop magazine actually tested penetrants for break out torque on rusted nuts. Significant results! They are below, as forwarded by an ex-student and professional machinist.

They arranged a subjective test of all the popular penetrants with the control being the torque required to remove the nut from a "scientifically rusted" environment. **

Penetrating oil ..... Average load

-None ..................... 516 pounds

-WD-40 .................. 238 pounds

-PB Blaster ............. 214 pounds

-Liquid Wrench ..... 127 pounds

-Kano Kroil ............ 106 pounds

-ATF-Acetone mix....53 pounds

The ATF-Acetone mix was a "home brew" mix of 50 - 50 automatic transmission fluid and acetone. *Note the "home brew" was better than any commercial product in this one particular test. Our local machinist group mixed up a batch and we all now use it with equally good results. Note also that "Liquid Wrench" is about as good as "Kroil" for about 20% of the price. *