8 3/4 Vent Blow-By

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Demon 408

Demon 408

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I'm having an annoying problem with my 8 3/4 blowing gear oil out of the vent when it's hot outside (80 degrees or hotter) and I run the car at speeds of 55-65 MPH or higher for extended periods. I've pumped oil out of the diff so it's about 1 inch below filler when cold, (car is on the ground/level), but it still blows gear oil out under the conditions I listed above. Seals are good at axles and pinion, no leaks. I used Mopar Sure Grip additive and NAPA Premium Performance Gear Oil SAE 80W-85W-90 (GL3, GL4, GL5) when I filled rear. Center section is not new but is a used one I had, it's got 3:55 gears and cone-type Sure Grip, rear end seems to work okay. Any ideas why this thing is puking oil out of vent?
 
Demon 408 said:
I'm having an annoying problem with my 8 3/4 blowing gear oil out of the vent when it's hot outside (80 degrees or hotter) and I run the car at speeds of 55-65 MPH or higher for extended periods. I've pumped oil out of the diff so it's about 1 inch below filler when cold, (car is on the ground/level), but it still blows gear oil out under the conditions I listed above. Seals are good at axles and pinion, no leaks. I used Mopar Sure Grip additive and NAPA Premium Performance Gear Oil SAE 80W-85W-90 (GL3, GL4, GL5) when I filled rear. Center section is not new but is a used one I had, it's got 3:55 gears and cone-type Sure Grip, rear end seems to work okay. Any ideas why this thing is puking oil out of vent?
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Is the vent tube/hose mounted in the correct location?
 
The only time I've had that happen to me was on a 71 Dodge PU with a 8 3/4 and it was because of excessive heat do to a bad wheel bearing. I think you need to take it apart and see what is causing the heat build up. Also by what your saying that it only happens when it is 80-up degrees in the day, this is adding to the heat generated by a bad bearing.
 
SGBARRACUDA said:
The only time I've had that happen to me was on a 71 Dodge PU with a 8 3/4 and it was because of excessive heat do to a bad wheel bearing. I think you need to take it apart and see what is causing the heat build up. Also by what your saying that it only happens when it is 80-up degrees in the day, this is adding to the heat generated by a bad bearing.
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I would agree, something is generating heat and causing the fluid to expand and generate excessive pressure.
 
Demon 408 said:
I have not measured the temp (infared heat gun) but after a few minutes (10-15 mins) the diff center is very warm but not burning hot (touch it with your hand).
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I think I would get an infared heat gun it out real good. I agree with what the others have said about something causing heat and causing your problems.
 
Demon 408 said:
I have not measured the temp (infared heat gun) but after a few minutes (10-15 mins) the diff center is very warm but not burning hot (touch it with your hand).
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I don't think I was clear with my description above, so let me clarify: After driving for 1 1/2 hours at 55-65 MPH, and then a 10-15 min cool down the center is very warm, but not "burning hot". Rear housing ends out by bearings are cool.

I appreciate the responses and I won't rule out heat/bad bearing. But, I'm not convinced, especially based on fluid level and where the vent is located. My next move will be to pull axles and third-member to check and re-lube.
 
I also emailed Cass at Dr. Diff the original question I wrote in my first post. I'm going to follow his recommendation and move the vent. Here is my email exchange with Cass and his answer:

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Hi Don,

What year is the housing? Is it an original A-body unit?

This is a common problem with pre '70 housings that have the vent bolt on top of the third-member.

The vent bolt should be located outboard of the third-member on the axle tube.

Cass

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Cass,

Thanks for the response. The rear is a 67 B-body that I have in my 72 Demon. The vent is in the stock location on the slanted part of the housing, I guess you could say it's at the 10:00 o'clock position on the third-member looking at it from rear of car. What are my options to stop it from blowing oil out of vent? Would a vent hose similar to what is on a 4-wheel drive work?

Don

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That is your problem. You need to move the vent hole to the stock A-body location on the tube.

I have never seen a vent hose work, even when it is attached to the frame rail.

Cass
 

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Oklacarcollecto said:
I think I would get an infared heat gun it out real good. I agree with what the others have said about something causing heat and causing your problems.
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I drove the Demon for 30 minutes today and checked temp of center section with a infrared heat gun, temp was 117 degrees. Not hot enough to indicate an over heating problem.
 
Demon 408 said:
It's not the B-Body rear that has the different vent location, it's the 69 and earlier 8 3/4 diff that has vent located on above center section.
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Yeah, I got that, that's what I have in mine. Vent is in the same location. I'm going to keep an eye on it for sure.
 
Demon 408 said:
It's not the B-Body rear that has the different vent location, it's the 69 and earlier 8 3/4 diff that has vent located on above center section.
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Thanks for sharing that information. You just saved me because I was going to weld the tube hole up and move mine on the housing like the factory did in 1968. Luckily both of my rearends have the vent on the tube.
 
How many 8 3/4 housing were made and used from 1969 and back? I really find that explanation hard to believe.
 
Oklacarcollecto said:
I wonder why there were changed in 1970 if there wasn't an update? I wonder if there was a TSB on this back in the day?
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Still Form 1957-1969 is a bunch of 8 3/4 housings on the road. I would think if it were a problem it would have shown up long before 1969. Not to mention the amount of pre 69 housings that I have run in my life without issue numbers more than a couple of few dozen.
 
SGBARRACUDA said:
Still Form 1957-1969 is a bunch of 8 3/4 housings on the road. I would think if it were a problem it would have shown up long before 1969. Not to mention the amount of pre 69 housings that I have run in my life without issue numbers more than a couple of few dozen.
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I don't disagree with you. I have also ran a lot of there rear ends mostly 1967 through 1969 in my youth and never had issues. Maybe there was additives available back in the day that thanks to the EPA we might not have now.
 
Just a guess and probably not a good one.

Would the fact that better modern oil that doesn't thin out as much as older oils be a reason for the different behavior of the oil near the pumpkin? Hence the reason issues show up on the near pumpkin vent design nowadays..
.
 
Rice Nuker said:
Just a guess and probably not a good one.

Would the fact that better modern oil that doesn't thin out as much as older oils be a reason for the different behavior of the oil near the pumpkin? Hence the reason issues show up on the near pumpkin vent design nowadays..
.
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I don't know, you raise a valid question. I have always used regular old 75-90W, Maybe synthetic oils are the problem?
 
In reading the TSB's for 1969 it appears that the differential gear oil was changed. The plate type Sure-Grips were replaced in late 1968 with limited amount of spring loaded cones. 1969 were all spring loaded cones.

"LUBRICATION

Lubricant required for axles with the new “cone” type Sure-Grip differential just meet MS (Chrysler Material Standards) #4893. Chrysler Part No. 2933565 meets these requirements. MS-1569 Chrysler Part No. 2585318 must not be used in these axles.
MS-1569 Chrysler Part No. 2585318 (or its equivalent) must be used in plate-type Sure-Grip axles. These axles have identifying tags stating, “Use Sure-Grip Lube” or “Use Limited Slip Lube Only”.

Conventional Differentials - Part No. 2585317 or 2933565
Plate Type Sure-Grip - Part No. 2585318
Cone Grip Sure-Grip - Part No. 2933565

If you should install a 1969 model differential and carrier in a pre-1969 vehicle and you require using the original drive pinion drive flange, it will be necessary to install the 1969 drive pinion seal guard on the original flange. This new guard is not interchangeable with previous model carrier castings."

I think I retyped all this correct. I have no idea if this will help here but maybe the different oil has something to do with the problem with the Cone Grip Sure-Grip. This is mentioned in more than one TSB in 1969. The differential gurus might know what to make out of this. It appears that the Cone Grip Sure-Grip was an issue. Maybe that is why the housing was modified for 1970. The 1970 TSB has only 1 page on the outer seal replacement on the 8 3/4.
 
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