Can a rear differential be 'fixed'?

My car smells a little like something’s burning and sounds like I’m engine braking when I take my foot off the gas (on the flat). My brother stated definitively that it was the rear differential - something about burning oil. I took it to the dealer who said it couldn’t possibly be from the donut tire I had to use to drive to the airport, but lo and behold - the rear dif was having issues. I read the manual for my 2005 Subaru Impreza very carefully when I changed the tire, in the dark, in my driveway - with a flashlight, and no-where in it did it list how far I could go on the spare. I ended up going about 50 miles at below the recommended speed because it was 3 AM and I had no other way to get to the airport. It did not mention anything about turning off AWD by removing or adding a fuse (as has been mentioned elsewhere in this forum) or any other way of turning off the AWD for that matter. It said NOTHING anywhere - I checked before, after, and again just now. Even when I called the dealer, AAA, and four different tire shops no-one could tell me how far you can drive on a spare or whether or how it might damage my car.

Anyhow I bought 4 new tires before driving home from the airport, but seem to be stuck with burned oil in the rear differential. The dealer acted like there was nothing they could do except replace it when it got bad (it’s barely noticeable now). I asked if they couldn’t just change the oil - to at least delay any additional damage but the guy shrugged as if it didn’t matter. Is that really true? Is there nothing to do but pay $2,100 to replace the rear differential. Seems crazy.

It’s a manual transmission and it did start snowing badly on my way to the airport if it matters (but I was driving very slowly - with hazards on and everything).

AND I DID READ THE MANUAL - really!!! There’s nothing in it!

I would get the fluid changed in that differential You don’t need the dealer, any mechanic can do it. Just make sure the right spec fluid is used. It will probably give the spec in your owners manual.

Hope fluid changes work, but you may be looking at damaged rear diff. or center diff.

Get all the driveline fluids changed; transmission, transfer case, front differential, and rear differential. Then hope for the best. Those temporary spare tires are really for getting off the road and to a tire shop ASAP. They are not designed to drive for 50 miles. AWD cars like a Subaru are even more touchy and there should be more warnings and more specific information than what you found in the owners manual.

On an AWD Subaru I’d go about 10-15 miles on a temp spare, and then only at 35 mph max. Better to call for a tow and take a cab to the airport.

I don’t know how removing the fuse would save the rear differential…the center yes. If the donut was just narrower and not smaller in diameter, it should not have been the cause. Yes, change all diff fluids…it can only help.

A smaller diameter tire on one side will cause the differential gears to turn at times when they wouldn’t be turning if the tires were the same (except briefly during turns). These gears aren’t designed to be turning all the time. This still shouldn’t damage it with just 50 miles, but maybe it was on the way out anyway. How many miles do you have on the clock? Over 100K and maybe the differential was on its last legs, and the smaller diameter tire for 50 miles did it in. If you were driving slow, 35 mph or less, it seems extremely unlikely this would damage the differential, but I guess it is possible.

In any event, a rear differential going out isn’t that uncommon. I wouldn’t lose much sleep about it. Just get it fixed is all. The time it happened to me was with an early 60’s Ford, and the symptom was that all of a sudden it felt like I was riding a bucking horse. I could’nt go faster than 35 mph b/c of the all the jolting from the rear end. The mechanic said it was the “3rd member” on the fritz or something like that. I never have figured out exactly what that phrase means, but that’s what they claimed they fixed, and it wasn’t overly expensive. I think they put in a used differential gear set from a junkyard wreck is all they did. Worked fine for years.

Before I did that however, I’d check the fluid level, is it up to the rim of the fill hole? If not, top it off and see if that fixes it. Whether it does or not, I’d still change-out the old w/new fluid and again check to see if it was fixed or not, before replacing the gear set.

You have a used car. Not new. You can replace your rear end with a used one. You might save some money. If you can find one. Use the Internet. Call a local used car parts business. Wrecking or salvage yard.

Some years ago I had strange noises coming from my Positraction differential on a Chevy rear drive car. It turned out the bearings were going. My shop replaced the bearings for $400 or so, and saved having to get a new rear end.

In view of the car’s age, if replacement is necessary, I would go for a used one since there should be enough wrecks by now to get a good used one.

Firstly, I commend you for reading the owner’s manual. It’s unfortunate that in this case it didn’t help, but you did the right thing.

You don’t need to use a dealer for this work. You’ll likely save hundreds or more by using an independantly owned and operated shop. The dealer will use only an assembly “purchased” through the official Subaru distribution system, typically 2 to 2-1/2 times the price of an aftermarket assembly and many times the price of a used unit, and the dealer’s per-hour costs will typically be higher.

I’ve attach a link for you to give you an idea of some rear differentials, although it’s an EBay link and I’m always wary of those. You can print it, but if an independant can find a used one locally that’s a great option too.

Sincere best.

I destroyed the rear differential in a Crown Vic by driving it 200 miles at 55 mph with a compact spare…I replaced the entire rear axle with one obtained from a salvage yard.

But first, find out what lubricants are required in your drive-line gear boxes YOURSELF. Then, armed with this knowledge, have any decent repair shop change ALL the fluids using the lubricants you specify. Then drive the car until circumstances dictate repairs are needed…Good Luck…

Since your car has a manual transmission the drive train would have a center differential with a viscous power coupling. BTW the automatic transmission Subarus have the hydraulic clutch coupling that can be disabled with the installation of the fuse in the appropriate location. Driving with a donut tire with a smaller diameter than the rest of the tires in any position will cause the viscous coupling to over work.

Have a shop check the operation of the center differential. Lift a front wheel off the ground and also lift a rear wheel off the ground on the same side. With the grounded wheels blocked try to turn the lifted front or rear wheel (parking brake ‘off’ and transmission in first gear). The other lifted wheel should turn the opposite way with difficulty. Alternately, one wheel could be lifted; release the parking brake; place the transmission in neutral; block one of the wheels on the ground; and try to turn the raised wheel. Again, the wheel should turn with some difficulty. The service manual will probably have a specification for the torque required to turn in ft-lbs.

If it is impossible or if it takes excessive torque to operate the center differential, that is the reason for your overheating rear differential. What you are getting is bind between the front and rear axle. Bind develops in turns where the front tires travel further than the rear tires. This bind and the extra friction of the front and rear final drives is the reason the car “engine brakes” excessively. The front differential would also be overheating due to the bind but it is cooled by the thermal mass, large surface area, and cooling system of the transmission and engine. But, the poor rear differential is stuck away in the back far from any cooling capability and the oil overheats. .

The center differential part is not inexpensive (~$550) and it takes approximately 1.8 service hours to R&R it. But, if it is fixed you probably will only have to change the lubricant in the rear differential.

I am wondering what the warning decal on the donut tire, spare tire cover, or spare tire well says. This is where the manufacturer placed the warning instructions. The warning I see regarding the T-type tire is to drive below 50 mph and only to get the original, full size wheel/tire repaired.

If I am way off the mark, please comment back. Let us know what the final outcome is on this. Hope this helps.


FWIW There are many vehicles out there in which the donut is narrower, but has the same rolling diameter as the regular tires. I know this from personal experience, not hearsay.

I’m not sure if that’s the case here, however.

Anyways, I agree with @dagosa. Change that diff fluid and hope for the best.

OK Really, I have 147k and thinking of changing differential fluid, How many of you have done it or let it go, thanks as I plan on it for this spring maybe.