Short version:
How hot should the rear differential get after a 30-minute highway ride (~65mph)? Should it be so hot that I can’t touch it, or should I be able to rest my hand on it and enjoy the warmth it offers?
2003 Subaru WRX, stock (no modifications)
180,000 miles
80F ambient temp
Longer story:
There’s a significant whining/whirring sound coming from the rear of the car that was barely-noticeable about a week ago. I had my buddy ride with me and crawl around the inside to confirm that it’s coming from the rear differential area. It gets progressively faster as the car moves faster, and I can even hear it as I come to a stop (down to ~10 mph) when it sounds like a soft grind or rub. The sound is unaffected by gear selection or brake application, and the only effect that accel/decel has is to speed-up/slow-down the frequency of the sound. It gets more-pronounced as it gets warmer, too.
I (stopped first) got under and touched both rear brake backing plates and calipers and neither of them are hot. Sure, they are warmer than ambient, but that’s about it. However, the rear diff is hot as hell. I can’t keep my hand on it after my 30-minute @ 50mph average (max ~ 70 mph) commute… it’s freaking hot. Should it get that hot?
So I mentioned that I had my buddy ride around w/ me, and we were only gone about 8 minutes at average speed of ~45mph. When we got back, I asked him to feel my rear end and tell me how hot he thought it was. He did and reported that while it was warm, it wasn’t as hot as I had described it [del]in my ad[/del]. He went on to say that he found the left side of the ham was warmer than the right. I crawled under there to confirm and found that it was indeed slightly hotter on the left, and was overall cooler than in my previous temperature-sampling event. From this data, and the sounds he heard while performing the ride, crawl, and listen session he concluded that it was a bearing and not the gears making the noise at the rear diff. I plan to lift and crawl under the car over the weekend for a better inspection.
What do you guys think is going on and should my rear be so hot and bothered?
A rear diff is inefficient. Not so much because of the differential aspect of it, but mostly because of the right-angle drive that takes the mechanical rotation of the driveshaft and puts it on a different axis. This typically involves a hypoid gearset, which has a lot of sliding motion between the meshing gear teeth, which limits efficiency. So it’s reasonable to expect a rear diff to get pretty warm after cruising at high speeds. It might take 15-20 horsepower from the engine to keep your car moving down the road at 25 MPH, which might mean ~1-2 horsepower (750-1500 watts) being lost to the diff as heat. Sometimes you can see on pickup trucks that the diff cover actually has cooling fins on it to help it dump waste heat.
This is part of the reason for the relative efficiency of front-wheel-drive vehicles: because their engine and drive axle are on the same axes, they can utilize an epicyclic or spur-gear differential - no hypoid - so the differential has much better efficiency than a right-angle-drive rear diff would.
Bottom line, a rear diff can get pretty damn hot, and it’s OK.
If you’re still concerned about your diff, you might want to pose your question on a Subaru-specific discussion board, where you’re likely to get more people with direct experience on your make/model.
Thanks for the info Elf. Makes sense about the heat coming from the friction forces within, so I won’t get too worried right-away but I will be inspecting farther.
Oh and **Gus **, I meant to put in the OP (though i missed the original edit window) that i had confirmed the oil level in the diff was where it should be. It was brown and dirty, but didn’t smell any worse than gear oil usually does. That’s a good idea to employ a “pedestrian”. I may do that here in the work p-lot before we all go home.
Did you have to pull the cover, or was there a drain plug? If the latter, was it magnetic? If so, did you see/feel lots of palpable silvery chunks (bad), or just a smooth black sludge (normal)?
It has been a a while since I checked but it seems like about 230 degrees was pretty normal after highway speeds. This is going back about 40 years but we wanted to make some comparisons once on some vehicles so instructed the drivers to pull straight into the shop when they finished their routes so we could take the temp of the differential.
There were 2 plugs… one high (fill) and one low (drain). I only pulled the high one and dipped a finger in there. The low plug has the magnet on it, so I’ll inspect it as i drain the oil over the weekend.
230F… that’s hot alright. And it aligns w/ my “hot as hell” assessment.
Update for anyone watching; I borrowed one of those laser-aimed, infrared thermometers and did some temperature sampling around the rear of the car.
After sampling about 4 trips, here are the ~average* temps I got after a 15-30 minute highway trip.
Left wheel housing where axle enters hub: 175
L, outboard CV joint: 130
L, inboard CV joint: 140
exhaust pipe: 255
L side of differential: 155
Center of diff: 145
R side of diff: 140
R, inboard CV: 120
R, outboard CV: 115
Right wheel housing where axle enters hub: 115
80<Ambient<98
Road temp, unmeasured
NOTES
*The actual temps may have varied a little, but these numbers represent the constant trend seen at each sampling.
Degrees are F.
The L, inboard CV joint runs above the exhaust pipe and the pipe is also tucked very close to the L side of the diff.
The L, outer CV joint is miles away from the exhaust, as is the L wheel housing.
CV joint temps measured on the surface of the “boots”.
Ambient temp had no noticeable effect on sampling temps.
My assumptions now
The location of the exhaust pipe relative to some of the measured components explains the high reading in those areas.
The L wheel bearing is raising all the ruckus. Would you agree that it’s a suspect?
It’s worth a check. Jack up that wheel of the car and check the wheel bearing by grabbing the top of the tire and rapidly shaking it in and out. ANY lateral movement of the tire is suspect. Make sure none of your squishable bits are under the car while you’re shaking it. I’ve had cars slip off the jack before.
I figure the fact that diffs usually take quite a bit thicker lubricant than transmissions and engines would tend to point out that they run pretty hot.
My pickup uses 5w-30 oil, ATF+4 transmission fluid, and 75w-140 differential lube, which at 100C, translate into roughly 9-12 cSt, 7.5 cSt and 25-40 cSt (where cSt = viscosity measured in centistokes).
You might want to double check your rear tires and see if they are generating noise, lots of rear bearing get changed because of noisey tires. I thought more about those temps. The diff guages we installed would max out at 240 degrees. I don’t remember anymore the actual temp they were running at.
Yea, either I’m not cool enough or i don’t speak their language (almost identical posts), but I hear nuthin’ but crickets chirpin’ over there. Just logged onto NASIOC, but need a few authentic, benign posts before they’ll open the garage door for me.
Also, I never want to overlook the wealth of knowledge / perspectives here on The `Dope.
ETA: just checked my post over at the WRX site and after 139 reads there’s actually one reply.
The gear oil in a differential can get pretty hot without hurting anything, but at least IME it’s not normal for a regular car or light truck differential to get more than a bit uncomfortably warm. Your WRX has a limited slip rear end, which can get get pretty hot during spirited driving with lots of turns (the STI version even has a rear diff temp sensor and warning light), but I don’t think it should be getting scaldingly hot during normal driving around town and during normal mostly straight highway driving.
Do you know the maintenance history of this car? The diff running hot could definitely indicate a problem with the internal parts, a fluid problem or even something as simple as a tire mismatch. I’d definitely give it some scrutiny, especially with the noise coming from back there too.
