Diff oil out the vent, why?

-
John Trickler

John Trickler

Well-Known Member
Joined
Jan 22, 2020
Messages
99
Reaction score
32
Location
Rolling Prairie, IN
I have a 742 pig with 3.90 gears built new for me 12 years ago. Installed it back then with gear lube. Just now getting the car to the driving point. It leaks gear lube out the vent cap. It is not over full. If anything, maybe a little low now. The car has maybe 10 test drives on it, and every time I have gear lube coming out the vent cap. Any ideas why?
 
Is the vent on the side of the hump or on the axle tube?

Is the vent new or old

If not too full, a fix would be to weld on a splash cover inside the housing like valve covers have.
 
Dana67Dart said:
Is the vent on the side of the hump or on the axle tube?

Is the vent new or old

If not too full, a fix would be to weld on a splash cover inside the housing like valve covers have.
Click to expand...
The vent is on the side of the hump, it is an old vent. If I end up taking it apart, the baffle idea sounds good. Thanks
 
My understanding is the hump vent tends to "leak" from slung oil, so they moved it to above the axle on the tube.

Mine is a hump style and an old used breather but I have not had any issues.

85w-90 Amsoil gear lube.

3:23 gears.

I wonder if the ratio increases the slinging effect.
 
There are only a hand full of reasons for oil leakage at the breather.

No particular order
  1. Too much oil
  2. Too light of an oil?
  3. Slinging from the ring gear
  4. No sealant on the threads.
  5. MAYBE... If it is clogged and the differential builds pressure from heat and then valve opens and a woosh of oil air comes rushing out
 
I had the same problem. Tried stuffing a bit of Scotchbrite pad into it, but it still leaked. I finally soldered an inch long piece of tube to it and put a length of hose on it routed up higher and pointed it down at the end. No more leak.
 
It’s a pretty common issue with the earlier vent location. Mopar moved it in ‘70 to be further out on the tube. I have an earlier one in my Duster and it definitely leaks oil. But if it plugs it pressurizes the housing and pushes the gear oil into the bearing grease and out past the seals and into the brakes.

Not my picture, but I’ll repost it as it’s pretty useful

Rear Diff Vent Loc.jpeg
 
Dana67Dart said:
My understanding is the hump vent tends to "leak" from slung oil, so they moved it to above the axle on the tube.

Mine is a hump style and an old used breather but I have not had any issues.

85w-90 Amsoil gear lube.

3:23 gears.

I wonder if the ratio increases the slinging effect.
Click to expand...
Makes sense, I may try to put a hose on it like a 4-wheel drive truck for now. I don't want to pull the whole rear housing if I don't have to. RMS suspension and Wilwood brakes.
 
Just make sure it's clear. I had the one on my 71 Jeep and it got plugged. It was off the road for a long time before I drive it. On long trips, it would push oil past the axle seal from the extra heat of the ride as 72bluNblu mentioned.
 
Demonic said:
Just make sure it's clear. I had the one on my 71 Jeep and it got plugged. It was off the road for a long time before I drive it. On long trips, it would push oil past the axle seal from the extra heat of the ride as 72bluNblu mentioned.
Click to expand...

RustyRatRod said:
What method are you determining that it's not over full?
Click to expand...
pull the fill plug out and it does not run out. The fluid is 1/4" to 1/2" below the plug. The ent is not plugged.
 
John Trickler said:
pull the fill plug out and it does not run out. The fluid is 1/4" to 1/2" below the plug. The ent is not plugged.
Click to expand...
That may still be over full. That's not the correct way to fill it. The capacity is listed in the factory service manual. Almost always without fail when you fill a differential using that method, it is over full.
 
Last edited:
You may have a short tube vent bolt on a hump brake line. If the threads don't extend past the thickness of the housing enough the oil will flow down the hill up the vent bolt hole and out.

Tube vent bolt threads are shorter then the hump style to keep them off the axle. See if the threads extend past the housing on yours. They should. This stops the leak, Been there. **** don't flow up hill. Is your replacement brake line thicker the the original?
 
jbc426 said:
I had the same problem. Tried stuffing a bit of Scotchbrite pad into it, but it still leaked. I finally soldered an inch long piece of tube to it and put a length of hose on it routed up higher and pointed it down at the end. No more leak.
Click to expand...
Thats how most pickup trucks and vans are.
 
Guess all 3 of mine are over full. I had one vent leak, I put flared fittings and a pigs tail 3/16" tubing in for the vent, works fine. Have a new vent, just keep forgetting to install it, been 2 years now.
 
Mine must be way overfull too, as she is 7* nose down at rest, and oil level with the bottom of the hole. When I gas it, the pinion rises towards zero, and now she's really overfull, lol. I just like that small front pinion bearing well-lubed.
I have 3.55s, and a hump-vent. and yes I installed a long hose on it. Been like that since year 2000 or so.
But I rarely exceed 65 mph, and so, with 27s the axle rpm is very reasonable.
 
For longer higher speed use, the factory fill level is appropriate. I've heard that for 1/8 and 1/4 mile use etc, you can get away with around 1 quart of gear oil, as that will provide adequate lubrication, shack-load protection and require less power to spin. I have not tried this, but there are guys & gals on here with vast knowledge and experience that may chime in.
 
You must log in or register to reply here.
-

Back
Top