I bought my first RX7 less than two weeks ago. It's a 1984 GSL-SE, and it's overall in decent shape but it has some minor issues. I've been able to track down and fix a lot of things, but I have a persistent whining/grinding noise coming from the rear end and I don't quite know what it is. The sound changes depending on if I am accelerating or letting off the gas, but it is not related to the clutch or the RPMs. It does get louder at higher speeds.

I have read every thread here about this and it seems like it is most often an LSD problem or axle bearing problem, so I took everything apart tonight. I seem to have some noise and a little play in both bearings, but the LSD looks good as far as I can tell. I do not see any marks on the gears and there is just a small amount of play between the pinion and ring gear. The pinion does make a slight clacking noise when I turn the ring gear. Is this normal, or is this a problem? I don't have enough experience with diffs to know, so I appreciate any input. I am pretty sure the axle bearings are bad, but I didn't think they would make that much of a grinding noise. I have a video of one of those too just to be thorough.

LSD noise:
Axle Bearing:
So, am I on the right track here? Anything else I can check?

 

Hi,
The bearings are definitely shot. They are dry inside. No grease left in them. A good bearing would not free spin like that.
You need to buy the 2 bearings, the collar ring and the oil seal that goes on the housing. Replacement is somewhat difficult.
The housing amplifies the sound of those 2 bearings a lot. It doesn't seem to me like the LSD is bad. The bearings might be but replacement is much more complicated than those on the axle and adjustment is best left to experienced people.
The LSD makes noise mostly when turning if the clutch plates are bad but not in a straight line.
Be sure to use new oil with the right LSD additive when re-assembling.

 

Thanks for the help, you confirmed what I was thinking. Now I just have to find a shop that will do the bearings for me (I don't own a press or anything).

I did buy Red Line Gear Oil to refill the LSD with. However... I messed up the rubber gasket when I removed the diff. Any idea where I can buy another one? It doesn't seem to be a standard part. I thought I could just order one off mazdatrix or something.

 

I have never one with a rubber gasket, I have only seen it sealed with silicone. That includes the Kia housing that I took apart for the gear.

I just replaced the axle bearings on our race car, it is much easier to press the new bearings and collar on than it is to press them off. I ended up cutting the old bearing and collar off in a mill, just machine it down until it is almost cut through and it will relax enough to slip it off by hand. Then pressing the new parts on is a piece of cake.

 

That's what it is... some kind of silicon sealer. It was black and came apart in a bunch of pieces so I assumed it was an old rubber gasket. That clears things up!!

 

Best stuff I've used for rear end silicone.

 

^ +1. Essential to use the gear-oil-resistant blend... the other kind will start to weep fluid after a while.

 

Thanks for all the responses. I'm going to order that RTV. I didn't know I needed a gear-oil resistant one. Now my challenge is to find a local machine shop to swap the bearings for me. No luck so far...

 

If you are going to the trouble of pressing on new bearings I would think about getting some heavy duty bearing retainers. The stock sheet metal retainers really flex and let the axles move side to side, they can cause some really funky noises under heavy cornering. On our car they flexed enough to rub grooves in both rear rotors when I had over 2mm of clearance when I measured everything statically. These are some I designed and had laser cut from 3/16 plate they work great.

 

Alright, I got new axle bearings installed but it has no impact on the grinding noise from the rear end. Is there anything other than the diff that it could be? The diff seems to work ok, maybe it is just noisy

 

Could it be the driveshaft?

 

I think it could be but I don't know how to tell, really. When I reinstalled the diff I slid the driveshaft back into the transmission and there was maybe 1/2" gap between the differential and the driveshaft, before I bolted them together. Is that normal? The u joints seem ok as far as I can tell. What else can I do to troubleshoot it?

 

Can anyone tell me how to easily check for bad U-joints? I assumed they were fine since they aren't obviously broken, but I don't know for sure and I am running out of options

 

Humming noise starting from low speed. Its best to remove the driveshaft and check both u-joints. If there tight or doesnt flex, its time to get another driveshaft since u-joints on 81+ are not replaceable. You can bring it to your local driveshaft specialist so then can chop and weld a new yoke + new replaceable u-joints. Last time i did it, it was approx $200 in LA

 

I have done more reading here and it just doesn't seem to fit the description of bad u joints. This is a definite grinding noise that gets worse the faster I drive and changes when I let off the gas. It really sounds like gears grinding more than anything. Even though my axle bearings were toast, replacing them has had no impact.

I had the driveshaft out a few days ago but I didn't take a close look. Guess I'll pop it off again and see. Maybe I can get an audio recording of this

Edit: I should say that the u joints were definitely not stiff. Both ends flopped around pretty easily when I was moving the shaft around and lining things up after reinstalling the diff.

 

Try rolling at speed in neutral, if the noise persists its probably your rear end if you think the driveshaft
is good. If it goes away its a tranny issue.

 

Thanks, but I have noticed already that the noise is constant even when in neutral or just depressing the clutch. That's why I am concentrating on everything behind the transmission. Is there a chance my diff is bad, even if it looks ok otherwise? There's a video of it out at the top of this thread.

I will try to get an audio recording once I figure out why it's misfiring... I'm working on two problems at once!

 

There is a sheet metal around the driveshaft's input shaft (tranny end). If its bent or smashed, it with grind against the tailshaft

 

I finally got under my car to do some investigating, and I stuck my phone up by the driveshaft to take some pictures. I found something interesting. Under the front u joint, there is what looks like scraping marks on the sheet metal. The whole piece is rusty, except right under the u joint. It spins freely off the ground but maybe with some torque it bumps into the metal.

I guess my next step is to get that metal off and bend it out around that spot or something. I will report back when I've gotten that done, just thought I'd share the picture:

 

Alright, I spaced the sheet metal about 1/4" further down using washers and the grinding noise is still there. I know the noise is behind the transmission since it is constant with speed, even if I'm in neutral, and after replacing the rear brake pads and checking the rotors as well as replacing the axle bearings and removing and inspecting the diff, I am stumped. The u-joints in the driveshaft don't have any play, nor does the diff as far as I can tell. I guess it just has to be my diff though? Should I just live with it until the diff goes out?

 

I'm not much of a good mechanic as the other guys but how are your break calipers?
are your pads getting uneven wear?

 

The pad wear is even and I recently replaced the rear pads just due to age.

After talking to some people I've been told the carrier bearings inside the diff could need replacing since the axle bearings were shot. I guess the axles could have been too loose and putting pressure on the carrier bearings in the diff. Is that a possibility and something other people have experienced?

 

Sounds to me like your transmission mount is going out, causing the transmission to sag and the shaft to rub the heat shield. I had that happen on my GSL. It's the center one that looks like a rubber block.

 

Hopefully you find/found the problem. Please post when you find a solution. Threads always end without a clear answer. Post for posterity!

Anyhow, after swapping in a manual tranny and going for the first drive I heard an awful sound screeching metal grinding on metal. I had bent an exhaust heat shield on install and it was grinding across drive shaft. I doubt this is your problem, because the sound makes it locating it origin pretty obvious. Recently after installing new struts/springs/packed lsd/gears/panhard i hear a noise that sounds like some washers banging loosely on the chassis. I think its the strut or the heim joints on the panhard

Have you jacked up the rear end and tried spinning the whole assembly by hand?

 

Unfortunately I haven't found the problem yet. I have put it "on hold" while I figure out some idle issues I am having.

Just to summarize, things I have tried:
1. Replaced rear brake pads
2. Replaced rear axle bearings
3. Removed & inspected LSD. Reinstalled with redline gear oil
4. Removed and inspected driveshaft. U joints seem OK
5. Spaced sheet metal down from driveshaft using washers. I don't think the sheet metal is my problem at this point.

I need to jack the car up and inspect the transmission mount, as LizardFC pointed out. I have indeed jacked up the car and spun it by hand, and I do not hear anything. There has to be some kind of load on the diff, either accelerating or coasting down a hill in gear, for me to hear the loud grinding. My grandpa is a mechanic and pointed out that if the axle bearings were in bad shape it's possible the carrier bearings in the diff are in bad shape too, but I would love to hear other opinions on that before I remove the diff again.... that is a pain to do with just a floor jack to work with.