On Toyota Tacoma and 4Runners, the axle oil seal will start to leak if the differential vent is plugged.
That little valve is located on the rear axle housing on the left side of the differential. It is a spring loaded, capped, and sealed valve that lets hot axle gases escape and keeps water and mud out. As the truck gets driven, the axle will heat up and the air inside will push against the spring sealed valve and hopefully escape to the atmosphere. If the gases can’t escape out of the vent, then they will escape past one of the axle seals, which forces oil with it. All Toyota rear axles have this same valve, but not all of them have persistent axle oil seal leaks…..
The valve needs to let the gases out, but it also needs to seal the axle and keep water out if the truck gets driven in water while 4 wheeling, or launching a boat for example. Dirt and corrosion eventually seal up the valve and keep the air from escaping, which sets up a chain of events for a frustrated Tacoma or 4Runner owner:
1: The owner or a shop notices oil coming from the rear axle housing, getting all over the rear brake shoes and tire. The owner then authorizes the repair which includes new brake shoes and an axle oil seal.
2: The owner notices that it still seems messy, or maybe the brakes lock-up easy on the affected side.
3: The owner takes the truck back to the shop to have them fix it again, they may be sold a wheel bearing, or have the outer seal replaced.
4: Repeat steps 1-3 until the owner pulls all of their hair out, and the tire is melted to goo.
That little vent valve should be replaced when the axles start to leak with an aftermarket unit, or someone needs to take the cap off and attach a hose to let the axle breathe. But that is only part of the problem.
A lot of shops that do not specialize in Toyotas will just replace the wheel bearing as part of the sale without thinking twice, but that is the problem! If you look below, you will see two axles. The one on the left with the notation of the distance needing to be 1/8″ is the one we are concerned about. That smooth collar needs to be no more than an eighth of an inch down past the machined part of the axle. If it is more than that, the seal that goes into the axle housing won’t have anything to seal against!
When Toyota started putting 4 wheel anti-lock brakes on the Tacomas and 4Runners, they set the inner axle housing seal closer to the differential to make room for the anti-lock tone ring, making that collar look out of place. The problem is that a lot of shops are used to putting the collar as close to the backing plate as possible when they do a wheel bearing, or when they replace the outer seal, and forget where it was when they pressed it off, which leads to the vicious cycle mentioned above.
So if you find yourself saying “nobody can fix it”…. Let us give it a shot!