What do you mean your diff oil was expired?

 

I should be more careful with my posts...

I meant "Expired" figuratively. The oil was dark...i.e. in dire need of a change!

And yes, gear oil can smell terrible. A few months back, when my problem started, I had bought a small container of motorcraft synthetic 80w90 to top up the gear box of a small machine. Initially I thought I had spilled some on the ground and got it on my boots - my first theory the bad smell in the cab.

A weird coincidence now that I think of it...

 

Interesting post. My '06 with 33k miles is smelling inside now. I just noticed it recently. At first, I thought one of the spare oil containers (engine oil, tranny, and diff oil) I keep under the rear seat was leaking. I checked all the containers and they are dry as a bone and I keep them each double bagged. I do not notice the odor while driving and there are no leaks on the driveway. It stinks and I'd like to know what it is. It's most noticeable upon first opening the truck after its been parked. pcm

 

Yup...mine was exactly like you. In fact, I even had Amsoil in my diff with the larger diff covers too. I had no leaks either, other than the sweat around the pinion seal - but no drops of oil on the driveway. Not even one. When the diff oil was changed, the smell disappeared. I really thought the diff oil would have lasted longer because it was Amsoil and I never drove the truck in 4 x 4.

The service manager at the shop even guessed it was a hose by the diff picking up diff fumes from bad oil. And he was right!

 

I recently changed my front dif fluid. This will be the second time since new. I thought the smell might be from the front diff, however, since the fluid change the smell is still there. I switched to Amsoil in both differentials at 15k miles and changed both of them again at about 32k miles. I do not see any leaks and the smell is still present and seems worse.
I can't put my finger on what the smell is but it's not pleasant and is strong. Very strong after the truck has been parked. Seems to dissipate after driving but returns quickly once parked. Leaving the windows down doesn't seem to help.
Lets say it is the pick-up tube near the differential. Someone please help me to identify the tube and what to do with the hose once I do so.
As always, thanks for the input. Phillip

 

I located the vent tube today. All looks normal. The end of it was greasy as was the inside of it. I do not believe it has ever dripped on the pavement - if so, very little.
From all I have read, this seems to be the source of the smell in my cab. What I can't understand is why the garage does not smell like the cab. I do not notice any smell in a closed-up garage. Only in the cab after parked a while. pcm

 

I get a lot of blowby smell in the cab too. Just yesterday I ran a hose from the breather tube all the way back to the rear bumper. No more smell in the cab for me! My truck has less than 43000 miles on it too...

 

Oddly enough, after I changed the engine oil and filters, the smell has gone away. pcm