tbear853

Every time since our purchase last year that I crawled under our 2008 GT there were a couple drops rear gear lube clinging to the snout.
I never saw a drop on the carport floor though.
I have checked the level a few months ago by pulling the fill plug just long enough to see there was plenty of lube in there waiting to get out. Since then we've put some miles on it with some several day trips and we went to CMS for the 50th.

A while back I backed her up onto my ramps, not steep ones, just enough for me to get under it. I see FoMoCo says 3/16" below fill plug. Mine is some over the fill plug still, I feel sure that it's the factory fill.
I have read a lot of posts on the net from Mustang owners having pinion seal replacements for leaks ....
.... and many others from those who find the factory fill like mine, above the hole.

I suspected the two are connected ....
.... and a spring loaded vent doesn't help.

I left the factory fill as is, minus the teaspoon worth that I let escape. I pulled the factory rattle cap vent (there is a rubber disc and spring loading it under that cap) off, 9/16" wrench .... and found the threads are 7/16-20 straight. I popped the top off, it's got a weak spring pressing a rubber piece down onto end of vent. I saved that.

I took a brass 3/8" hose barb that was threaded some larger and turned it down and used a die to cut 7/16-20 threads . It's near an inch from the outer edge of axle tube to axle shaft inside, so I'm good even though I have a longer threaded portion. Temporarily I had a piece of 3/8 hose clamped to it and clamped to a U shaped piece of copper tubing. 3/8" OD with open end pointed downwards.

Well, since then . ... with just the fitting to hose and a 180 degree piece of copper, no vent restrictions .... "Wife Unit" and I took the Mustang on a drive down to far SW Virginia to see family and I'm guessing it was near about 250 miles 'round trip.

Saturday morning I backed her up onto a couple ramps on my "shop apron" and slid underneath for a look. That was the first time I did so and found NO drops of gear lube hanging on the bottom of the differential housing behind the pinion flange, still as clean and dry as I left it after removing factory vent and adding my temp vent. Checked level, there's plenty in there.

So .... I'm convinced that the factory vent cap with spring and rubber disc acting as a one way valve was building just enough pressure to maybe "encourage" some passage of gear lube past the pinion seal when warm.

So I left the short piece of 3/8" rubber clamped on the hose barb fitting I described (uses 7/16"-20 thread) and above that, a 180 degree bend of 3/8" copper tubing positioned so it would miss the exhaust or frame on full load .... and attached a length of hose on the down pointing open end and ran it over into the hole of the frame so it's not subject to water spray or windage. It's also pointed still downwards inside that frame rail so no puddles can develop in it.

I'll continue to check it when servicing ....
.... but unless I see more drops hanging on, I'm calling it "fixed".

tbear853

Updated to say we have put more miles on her and still NO drops of grease hanging under the rear axle ......

S2kpuddydad

Sounds like the factory vent was not venting well

As you now know pressure will cause a leak as well as a over fill. Good luck

tbear853

Not sure what's under my '01 MGM or my two T-birds for a rear vent, but the Mustang's OEM vent had a spring in the vent above the round rubber sealing disc and while the spring is not a stiff one, the opening for the vent is small .... so I think it likely it could build several psi real easy.


Took her ("Wife Unit" in Mustang) out Friday evening over to Low Moor for supper and back, roughly 90 miles 'round trip ... and then yesterday morning to a meeting in Harrisonburg, about 100 miles 'round trip ..... another 190 or so miles total last two days .... and no sign of gear lube leaking yet.

S2kpuddydad

Most differentials use the spring type atmosphereic vent, the other type is a captured system. The captured vent usually uses some type of vent box and a rubber hose that goes to the top of the differential held by a barbed vent tube.
Basically what you made your self.

tbear853

I wouldn't have thought to call what I did as "captured". It's just essentially a air gap in top of the loop to prevent water intrusion if ever submerged .... but maybe that's "the captured part" of it?


Anyway .... just returned to say several more drives, couple hundred miles more .... and still .... no more leaking!