Groaning diff

[onebluemcm]

Car has 32k miles on it, a lot of garage time. The diff had been groaning for the last probably two years, only 2-3k miles probably, it's not driven enough, I know.

I just got through with a bunch of regular service - engine oil, trans fluid, diff fluid, brake fluid flush. On the recommendation of Turner, I went with the Liqui Moly LS SAE 75w-140 synthetic diff fluid.

Initially, it's groaning every bit as much as it was with the old/original fluid. I haven't been able to go somewhere to just drive a lot of tight circles yet, hoping that will help get the new fluid all through it.

Wondering if from experience anyone can give an educated/semieducated guess as to if it's likely to get quieter with some mileage? Should I consider a friction modifier additive at some point? I don't want to diminish the LS function of it.

It hasn't, and isn't, doing any chattering, jumping, jittering, just the groaning and it seems smooth. I can't believe the diff could be shot at just 32k, but is that possible? Have I killed it from letting it sit too much?

Thanks much.

[GuidoK]

How do you know it's the diff that is groaning?

[onebluemcm]

It's 100% the diff. Only does it during very tight turns, typically a low speed, in parking lots etc. It happens at time when there is the greatest differential rolling speed between the rear wheels, thus the tight turns. It's a diff groan, what else can I say. What else could it be? Car is otherwise 100% and I have been all through this car, top to bottom, front to back. It's not a brake issue. It's not a suspension issue. It's not any other part of the driveline. You are making me second guess myself, but I shouldn't, it's the diff.

[Digs]

I have a very similar story to yours, same mileage and diff started groaning recently. I've done all the tight circles, round and round and back the other way round and round making myself dizzy and it didn't seem to improve it much if at all. But recently went on a few longer drives to get it properly warm and then did some tight circles and it was blissful silence!! My respected local BM shop guy has an E46 M3 and he says oh yeah they can make a little noise until they warm up. Hopefully it doesn't get worse because it's not a pleasant sound! Maybe we need to go on longer drives occasionally. Try getting it warm then do some cycle work.

[GuidoK]

Originally Posted by onebluemcm

What else could it be?

Tyres can also be a culprit

You are making me second guess myself, but I shouldn't, it's the diff.

Normally you diagnose this stuff with an engine stethoscope (really cheap tool).
So car on the lift, engine running in 1st, slow 1 wheel down and listen to the diff with the stethoscope.

Let's hope it's not your diff, because if it's as bad as you say, some different oil probably is not gonna solve it.
With 32k your diff should be worn in, and I assume your recent oil change is already suited for clutch style LSD's (so it already contains the necessairy friction additives, I believe the LS (limited slip) oil has this)

The only thing I can think of is that because you run your car so little, condensation has been forming inside the diff, and rust has set in on the gears. But not having rust on crownwheel and pinion but having rust on sun gears is not likely.

[dre99gsx]

Get some stuff out of the way

- any groaning on straight line deceleration?
- any groaning on full throttle?
- do you get same groan when cold and when hot? (it takes quite a bit of driving to get the diff hot..)
- if you only hear groan when turning slow speed tight turn, does it do it when you are delerating and turning, or slightly accelerating and turning?
- Try swapping left/right rear wheels (out of curiosity)
- do you notice any clunk when you give it a bit of gas, let it coast, back to some gas, etc?

I've had experience with groaning on decel and accel, and both were indications of bad pinion-ring mesh, eventually followed by the diff blowing up (racing and overheating).

I don't think that's your situation, so it seems more like something else.

Also, check your rear wheel bearings - lift rear, parking brake disabled, any play vertically and horizontally? (grab wheel top and bottom, or side to side, and see if you are able to rock it).

[Westersund]

If you rarely drive the car I would say that there's not enough lubrication between the friction plates and that's why it's noisy driving slowly in tight circles, if the diff is noisy driving straight you've got a problem. Get the oil in the diff up to temp and check if the problem is still there.

I changed diff in my E46 M3 to a real racing diff with 12 friction plates, before there was enough oil between the friction plates it sounded like the prop shaft was going to jump out of the car when turning in a tight circle. Now it's perfect. In your case it's not as extreme but I think that the "problem" is the same. Adding friction modifier will help with the groan but will reduce the effectiveness of the diff.

[onebluemcm]

Thanks gents. Ya, definitely no sound in a straight line, no clunking, no gear vibration, no noises that are constant or change pitch with vehicle speed. In the past it does seem to improve or disappear after the car has been driven for a while, I haven't put a time on it yet, but yeah, more or less thoroughly warmed up.

I just flushed brake fluid, in addition to the diff fluid, also changes trans fluid and engine oil, and am installing Bilstein PSS10 system on it, so I've had a close inspection of everything including wheel bearings, wheels, tires, all fine. The diff case did have some fluid on it, possibly from the pinion seal but also possibly from the drain plug, it was hard to tell for sure of the source, so this is something I'll be monitoring. The fluid that came out certainly didn't look like new, but it wasn't awful either, and no sign of water per se, but microscopically, certainly possible. I highly doubt there has been enough moisture in there to cause rust.

Also, it's garaged in an environmentally controlled space, so while it's sitting, it's not experiencing big changes in environmental temps/humidity.

[wohcrag]

I thought the e85 didn't have a lsd ?

[Westersund]

Originally Posted by wohcrag

I thought the e85 didn't have a lsd ?

Z4 M does. Z4 Non-M doesn't.

[onebluemcm]

Little update - new fluid in and a few hundred miles on it, it's getting better. I'm not sure it'll be silent, but I'm happy that it's been improving with no issues with function, which I figure an only be a good sign.