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Leaky differential? Would oil wick up dirt?

Remounting my spare I noticed that the differential is oily. I scooped a bit up with finger. It seems to come out of the drain plug, but it goes up to just short of the filler plug and a few inches forward. It’s dirty, of course: would the oil have wicked up the dirt? Would anything else make this? There’s no oil on the pavement, even though it’s been parked for months. I changed it in 2000, 30K miles ago. It’s 80w90. Could it be a leak elsewhere? I think of a differential leaking as nearly impossible, at least other than the plug gasket failing.

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I’m not sure what you mean by wicked up the dirt. I imagine dust stuck to the oil as you were driving.

The pinion seal could be leaking if the oil originates from the front and trails back.

I’ve seen oil seep from where the axle tubes enter the “chunk” or “pumpkin” (technical term is differential I guess).

Obviously oil can leak from the differential cover.

Or I suppose the oil could seep from around the drain or fill plugs.

I’d suggest cleaning it up with some brake parts cleaner, then monitoring it to see where it’s leaking.

Also, you should be able to tell for certain whether it is indeed gear oil or not by the smell. Could possibly be hydraulic fluid from brake lines in that area. Can easily rule that out if it has that unmistakable gear oil aroma.

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The differential has some sort of a vent.

If it didn’t, when the gears/oil heat up the pressure created would blow out the seals.

Make sure that vent isn’t plugged.


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It was ambiguous. What I meant is ‘could oil that leaked out of the drain plug have been absorbed by the dust that was already there and flowed up to points higher than the source?’

Hmmm… I don’t know what these are.

The fill plug is above the fill level. One tests fullness by sticking one’s finger down the fill hole. How could it leak from that? It has been parked on the flat all this time.

It takes so little oil I’d worry about running low - and oil this heavy leaks so slowly, especially when I don’t drive.

You can maybe.

Up top? Where can I find a picture. I don’t have a lift; I lie on the ground on my back and whack my elbows on the chassis.

That makes sense. Then again it’s heavy oil and doesn’t get that hot, does it?

'Cause if it were it’d be forcing oil out the drain plug? Wouldn’t that hold? It’s a big bolt, attached tightly.

When the gears are turning inside the differential, fluid will be churned up above the fill plug. If it’s been parked since you changed the fluid (if I understand correctly), I doubt it is leaking there.

The pinion seal is on the front of the differential, behind the yoke (what the driveshaft connects to).

Tester is correct on checking the vent also. There should be a rubber hose leaving the differential and going up.

But if it isn’t driven, the oil won’t get hot, and it shouldn’t leak out of the vent or the fill plug. Is / has it been driven at all since you serviced it?

You can remove the fill plug, stick your finger in there and check the level, I know. Now, smell your finger! That’s gear oil. It’s rather distinct.

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I changed it in 2000; I’ve driven 30K miles since.

I still have a little left over from 2000; I can smell it. The stuff I got off with my finger didn’t have that smell and it wasn’t hard to clean off.

What path would the brake fluid take to get there? The level in the reservoir is okay. Wouldn’t it leave a puddle?

It could be engine oil, transmission oil, power steering fluid.

It is not unusual to see a dirty rear axle on an older vehicle. Check the fluid level and clean the housing. You could be losing as much as 1 ounce every 5 years.

The answer to your question is yes. The differential housing gets dirty as you drive around. If the oil leaks anywhere, it will wick through the dirt in all directions, including upward. BTW, it only takes a few drops of oil to look like a big mess on the differential housing. If it has been sitting for awhile, I’d check the oil level before driving anywhere.

I’m assuming this is a rear differential, correct? If so, the only fluids in that area would be brake fluid, gear oil, or gasoline (which would evaporate and you probably wouldn’t see). Brake lines are usually running along the top of the axle. Usually there’s a junction block nearby where a rubber brake line joins the two steel brake lines on the axle - the steel lines run to the brake cylinders on each wheel.

Like others said, I’d just check the fluids and clean it up if there’s nothing obvious. Then monitor it the next few drives. Losing a little gear oil will not hurt. If it isn’t an obvious leak you’re not going to run it dry very quickly. Most differentials I’ve changed fluid in hold over 2 quarts, maybe closer to 3. I’m not sure how much a Toyota holds, I’d assume close to 2 quarts minimum, so a couple of ounces seeped out isn’t going to destroy it.

Check the master cylinder just to be safe. In the event that it’s brake fluid leaking - much more serious issue. From your description of the area of the leak, it sounds like a minor gear oil leak.

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How would engine oil or power steering fluid get to the back of my truck?

Yes, the only 1 I have.

1.9 quoth the manual, and about how much I put in.

How could it leak minorly?

A minor leak doesn’t drip. Some manufacturers won’t even warranty a minor leak. No drip, they don’t pay. I’d agree, if it leaks, I’d want to repair it. Until you determine where the leak comes from, there’s not much you can do to address it. If it’s seeping out around the fill plug, for example, it won’t cause immediate damage. While I’d want to stop the leak, I wouldn’t be concerned about catastrophic damage, given the fluid levels are good. A few ounces low won’t kill a differential, just like a valve cover leak won’t kill an engine. I won’t stop driving a vehicle because of a valve cover leak. But I will monitor the fluid levels, clean the area, and once verified that it’s a valve cover leak, change the valve cover gaskets. For example.

IIRC, my 1979 Toyota truck’s differential had one or two vent holes. They would have had to have been above the level of oil inside. I remember preparing the vehicle for possible immersion in deep water inserting a barb into the hole and running a flexible vinyl tube from there up to a higher place and securing it there, with an in-line fuel filter at the end to keep contaminants out.

At 60 mph you would be surprised how far the wind blows a drop of oil.

That’s right

You just pointed out the difference between seepage and leakage

seepage . . . minor “sweating”

leakage . . . drops forming, and possibly drops and/or a pool of fluid on the ground

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I’d still be angry if my brand new vehicle weeped, though, right? A seal is compromised either way. An easy “out” for warranty work. Pointless in the discussion of troll’s 30 year old truck, but worth mentioning.

I scrubbed up the differential today. The leak smells like gear oil. This time I found a spot on the pavement underneath. When I drove it last month, for the first time in 2018, I made a point of inspecting the pavement underneath it for any signs of a leak and found none. I drove only 6 miles, uneventfully.
The leak looks worse on the front face of the differential; oil has moved nearly a foot forward. Unlike the differential in the video to which @It_s_Me pointed me, the body is attached on the front, not with a back plate. Could the leak be around the mounting bolts? It looks like it is.

I scrubbed it well and will check in a few days for a better idea of the leak.

@Tester: is the vent that thing that sticks out on the top, cylindrical and about an inch long? I drenched it in solvent and scrubbed it; the paper towel didn’t get very dirty, and not at all oily. I don’t see how I can get a better look on my back.

What’s with the 24 mm bolt for the differential fill plug? I shined up the 12-point socket in 2000 so I bought a hardened 6-point impact socket. I can’t find it; I figure it disappeared in the theft a few years ago. Is there any reason not to replace it with something smaller? I think I replaced the drain plug with a 17. The heads of the 24s are so thin.

When you replaced the gear oil, did you remove the cover (the back plate)? You said yours doesnt look like the only in the video. I’m not familiar with Toyota differentials, but most look pretty similar. It could very well be leaking around the bolts that hold the cover, especially if a mistake was made reinstalling the cover.

If you removed the cover, you should clean the mating surfaces before reinstalling it. I use brake parts cleaner and a rag. Run a bead of black rtv around the edge of the cover where it will mate to the differential. Go on the inside of the bolt holes, otherwise gear oil could leak around the bolts. 1/4” bead should suffice. I apologize if I’m telling you things here that are obvious to you.

Gaskets are available for some differentials. Some came from the factory with just rtv. I’ve always just used rtv without any leaks or problems.

The ‘back plate’ isn’t a plate, it’s a ‘tub’, a 3-D part that includes the horizontal faces as well as the rear. I haven’t removed it ever. 18 years ago I changed the oil. Since then I’ve used it for no purpose other than to make my rear wheels rotate at the proper speed. It’s never misbehaved or made noises.

If a mistake was made installing the cover it happened before 1998; I think it hasn’t been installed since when it was built.

If I have to rebuild it I’ll hire a pro. I think it’s only leaking. If it really is leaking around the mating surface and/or through the bolts, would applying RTV on the outside plug the leak?