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Rear differential problems - 2004 Forester XT Auto

Elaine Atkins

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Hi All

I was loving driving my newly acquired old Forester (immaculate, new MOT and service) until  2 weeks ago when it developed a horrible rear diff noise on tight turns - one month out of warranty . . . garage replaced the diff oil and  put in an additive and said run for 2 weeks and see if improves.  It hasn't.  I'm guessing this will be very expensive to repair and am not sure whether I should take a risk on a second-hand part - assuming I could find one (and a mechanic friend is even saying get a whole new back axle!)  or go for a new one.   Or  perhaps ignore it and hope for the best because funds are pretty limited after the car crash (not my fault) which wiped out the Jeep before I bought the Subaru as a replacement!  

Anyone have experience of this problem and if so, wondering what did you do to get it fixed successfully?

Thanks in anticipation of your replies.


Edited by Elaine Atkins
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Rear diff is a sealed viscous lsd unit so changing rear oil or adding additives makes little difference when this the fault .

Other thing check here is tyres are matching make, even tread wear and pressure correct. Any of this wrong and rolling circumference can differ enough to wind up transmission .

If when doing tight slow manoeuvres you can here and more importantly feel it in drive train DO NOT leave it as can seriously damage drivetrain to point of severe failure .

Is quite common see cars with rear lsd fault have recent new tyres and rear wheel bearing in effort fix the symptoms then sold as wallet burned out by bad mechanics/advise so see if this seems case with yours (what tyres on it and any recent bills or signs of work on the rear)

Rear diff if is needed best done with a used unit, XT diff is same as some impreza and not hard find and not expensive (under £100)

Removing and fitting rear diff not that hard of a job, about 2hrs if have few rusty fittings but simple enough and nothing complex.

Absolute no need go whole rear axle that just silly, be logical do some checks such as tyres wheel bearing and brakes, (infra red heat gun superb tool for this)

If doing a diff get someone who experienced with subarus and take bit of time source diff rather than blind panic buying first thing find.

New diff will be silly expensive so don't even bother unless was warranty assisted repair .

I would expect a small refund from dealer who sold you car, even if just £100 as good will gesture .

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If it mainly noise it may stay like it for ages but if can feel the issue at low speeds it really needs sorting .

I done the odd rear diff so I know the viscous units can go bad

Is your XT auto or manual ?

your first thing is accurate diagnosis or at least crossing off options narrowing it down.

I've had customers in who spent hundreds on rear wheel bearings and new tyre set then moved on to me for help and it was the diff.

flip this scenario and some tyre issues wheel bearing and rear brake issues can causes drive line type faults/noises so it needs looking at by someone with subaru experience.

I could probably diagnose that in 30 minutes from short test drive and putting it on lift and running it in gear on the lift and doing basic examinations, it not that complicated to a clued up mechanic .


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  • 1 month later...
  • 2 months later...

Excellent info Mr B!

 Recently, experiencing loud humming from the rear, gets louder with speed. Guessing it's a rear diff issue and I do have a laser thermometer. Any tips on diagnosing possible failures? 

The guy's I've been getting used impreza parts from could surely help me if the diff does need replaced. Which imprezas diffs match the xt auto diff and is there any upgrades to consider?

Thanks in advance


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Check wheel bearing temps with laser and also around the diff housing and see what pick up from that .

Don't jump to conclusions as bearings a common source of some serious noise, hopefully temps make something obvious, if having hard time pinpoint it get it on a lift and run it in gear on lift and listen for noises and check bearing play while wheels off ground .

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Sorry to Hijack the thread slightly but it may help.

Mr B when you say wound up diff, could you explain further.

I recently pulled the engine out my SG XT as the timing belt snapped and it needed many new internal parts, on putting the engine back in my rear right wheel seems to be locked. Wasn't brakes completely dismantled them and took disc off to check) and probably wouldn't be bearings as when it went into the garage their wasn't a problem. It was pushed around without the engine in to make some space, would this have caused the diff to become wound up? 

And if so, does that mean its new rear diff time? or can you  'Unwind' the diff?



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What I said was transmission wind up, this is when likes of poor wear matched tyres or tyre with low pressure causes rolling radius of tyres differ too much thus the different rotational circumference causes the props and driveshafts to fight against each other as they turning at too different speeds that the 4wd system can not handle.

Rear bearings pretty common cause of noise so check them out thoroughly .
Only way we pick up lot of bearings is via running in gear on ramp and with stethoscope or long screwdriver on rear hubs .

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