Gear oil smell after highway driving

Recently drove about 100 miles at around 70mph for the first time in a long time (I don't drive the taco much because of WFH). After getting back into town I got pretty strong whiffs of gear oil from inside the truck.. Looked for leaks and couldn't find anything. Poked around on TW for similar issues and almost all similar reports were after a gear oil change whether that was t-case, trans, or diffs. I haven't changed any gear oil since I bought the truck..

should I be concerned about the smell? Are there some things I should check on? I've driven the same route before without any issues or smells, this is the first time.

For reference, I have an 06 v6 6mt 4x4.

 

Are the rear bearing seals good? Would be leakage near the rear brakes.

 

Inspect the rear fender wells for gear oil.
Then let the truck sit over night.
Before you move the truck agin in the morning, look at the back/inside of each rear wheel.
Also look at the the bottom of the drum for gear oil leaking out. If it’s bad enough, it will be running down the inside of the back tire.

If it’s the front differential. It will be where the CV axle goes into the front differential.
Or it will be dripping down off the front left upper control arm. (From the diff breather)

FWIW, if there is a leak. It’s usually on the driver side. But check both sides.
Also, the rear diff breather almost never leaks. It blows the axle seal.
Why? - Because it plugs up and the only other way to release pressure is THROUGH the axle seal.

 

Would be my guess.

 

I had an issue like this with an old 5 speed hardbody, the gear oil residue on the trans made its way through the shift boot into the cab
I replaced the boot and got rid of the smell

 

Could it be you ran over some and it’s sprayed onto the wheel wells? Highways are nasty catch pans for the American public

 

If you haven’t driven for quite some time it could be accumulated excess moisture in the differentials/transfer case that evaporated and vented from the breathers carrying the distinct gear oil fumes with it.

 

When I had my 2006 I had the dealer replace the clutch. Gear oil smell in the cab afterwards. And you could smell it outside also, but not as bad.

Smell was bad with AC on recirc or with window cracked. Windows up and AC on fresh air and I couldn’t smell it.

Turned out the rubber shift boot on the trans itself wasn’t put back on. Smell traveled through the body boot and into the cab.

If it’s had the clutch done it’s worth checking.

 

Check like others have said the axle seals . Most likely it’s the rear .
Pull the rear wheels , and drums and you’ll know right away

 

Great, thanks for the info y'all. Will start by checking out the axle seals then look into some of the other suggestions!

 

Possibly a ripped CV also, simmilar odor. Ive had a torn CV boot sling grease into the engine bay and the exhaust downpipe, once it heats up and goes everywhere you get the same smell

 

I like this as a possible cause.

Usually I drive moderate distances with moderate loads, never really heating up the rear diff or the manual transmission that much. Recently took the fully-loaded (~1000 lbs of cargo) truck from 0 to 6700' elevation in the CA Sierras, pushing it quite hard all the way up the hour-long grade from Yuba City to Truckee on route 20. ~3.5-4.5K rpms, 60-70 mph all the way. Stopped just before Donner Pass to check the diff & trans temps by touch; both seemed very hot, maybe 150ºF or so on the outer case (by finger-touch estimate) at ambient temperature of about 64ºF or so.

Definitely smelled a mild odor of hot gear and/or transmission oil. Outside the cab, walking around the truck. Not inside.

 

If you're smelling it inside the cab I would check to see if the boot that seals fluid on the top of the trans isn't torn or unseated.

 

This too. I had a similar experience with the associated smell.

 

Take a look at the front diff breather under the hood.

That's my bet.

 

Check the levels of the tcase and diffs. there is a fill plug for each and the fluid should be level. Did you change the gear oil when you originally bought the truck? If not maybe do that as a safeguard.