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2015 WRX Rear Diff Problem - Please Help

7204 Views 8 Replies 3 Participants Last post by  mycologist
Hey All,

So, I've managed to anger the drivetrain gods in attempting to provide them with fresh blood and longevity.

So, I changed the fluids on my transmission and rear diff for a few reasons:
I drive the car nearly 40,000 miles a year and have a goal to get 300,000 miles out of the car. So to me, fluids are a bit up there on the charts to change often and keep at proper levels

I have seen many many reviews online for multiple non-oem fulids and how they help with smoother shifting, less drivetrain shock, and overall performance of the vehicle.

So, after changing the fluids (Transmission first, followed by a fault free test drive) Then the Rear diff, I noticed there was drivetrain noise coming from the rear end. This only happens under negative load (Not accelerating / downhill coasting or hard downshift ) And slow, hard turns or slow passes over bumps (The last one throws me off a bit)

Currently, I have tried changing the fluid with another brand, and have seen no progress. I am honestly baffled and have no idea where to specifically look or what to look for at this point. I have never had any noise that I could hear to this extent prior to the fluid swap.

I can provide pics and am even tempted to go as far as taking the rear diff cover off (not before buying a new seal) and inspect for excess play, or visible damage.

Any input, suggestions, or "Why the hell did you do that stupid"'s are appreciated.
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Fluids Used (Both XMSN and Rear Diff.)
Royal Purple 75W90, IAW Subaru owners manual for oil grade / weight / quantity.

New fluid in Rear Diff little improvement in noise:
Lucas Oil 75W90 Trans / Diff Oil.
So I had Transmission issues with an 87 Accord. I recall I had added some store brand transmission fluid for some reason. Then I drained and replace all the fluid with the store brand. Issue was the same. People told me Honda's needed Honda fluid. I drained the store brand stuff and filled with Honda fluid and my problem went away. Could the WRX need Subaru-only fluid?
I don't have experience with those fluids, but there have been issues with syncros with various "cocktails" and synthetics at least on older cars - they feel better at first then start to grind. There is a big thread here somewhere. What you want really (at least in the past) is the Subaru ExtraS. It is the best and safest. I use regular valvoline dinosaur gear oil and recommend either that or the official good stuff.

I've done a lot of bushings and I hear a lot of noise. In general it is to be expected and not necessarily a fluid issue. I've heard stuff going downhill engine braking that made my passenger think we were blowing up. This has been going on for 70K miles. My impression is that it gets worse when the tires need to be rotated. Maybe your fluid change is just coincidental to that or it may just be getting a bit more pronounced with some miles on it. Generally it is a steep curve to learn what to ignore on Subis noise wise - both the motor and the driveline tend to make an ungodly racket. Your are most likely fine as far as the diff. but IDK if I would keep that in the trans.
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Thanks for the Replies Joe and Cologist,

Depending if it doesnt start gradually getting quieter by some form of magic, I may give the Subaru brand a try. As far as keeping the current fluids in..... I'm probably going to do a oil analysis within 10k just to keep tabs on it.

When i did change the oil prior to adding the new fluids, there was a fair amount of shavings on the magnetic plugs which granted is expected as that was the original fluid since break in. Gonna keep a close eye on everything and ear to make sure nothing gets worse.

Up shot, MPG went up by around 2 MPG average. We'll see how long that lasts for..... I may also look into bushings as you say which may just be good timing.

Coming from an aircraft maintenance background, any new noise after fluids is a bad noise. So, it is entirely possible im making this bigger than it needs to be seeing as subi's have their quirks.
Also, will probably be getting tires rotated here this week and getting a nail patched. Might even just be a combination of everything all at once. But, who knows. Will keep this thread going as a log for anybody who has the same issues.
Also, will probably be getting tires rotated here this week and getting a nail patched. Might even just be a combination of everything all at once. But, who knows. Will keep this thread going as a log for anybody who has the same issues.
Yeah, please let us know.

Things inspected / adjusted:
Tire Pressure - Rears were at 30, inflated to 32.
Exhaust - Adjusted midpipe / resonator spacing, muffler spacing adjusted. Exhaust has .5 to 1 inch of space between major components and framing / head shields.
Rear diff oil - Inspected fluid level (Prior fill was done at a near level state, backed up onto ramps and re-serviced rear diff.

Vehicle was taken to a local shop and test drove with a mechanic for approx. 15 mins after car was fully warmed up. Does not think it could be a bearing and noted the sound level of the gearbox / transmission noise. Belives it could be suspension related (Sway bar, end links, bushings, or struts)

Monday vehicle will be put onto a lift and driven / investigated further with noise detection equip. to pinpoint possible location of noise.

Left rear strut has evidence of possible slow oil leak, found dried oil with mix of dirt on base of the strut. Noticeably softer when pushing down on rear of vehicle with trunk open on Rear left vs rear right. No visible cues to bushing deterioration and or failing joints.
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My idea would be to just drive it unless you are having an issue other than what you hear. Subarus are noisy - I'd be leary of a quiet one.
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