1999 GMC 4WD Sierra

Reader Question I have a 1999 GMC Sierra, 4wd. Just recently, after towing a trailer, we noticed a smell from the rear Differential. The axle housing was very hot, yet we noticed no noise that you would expect from a bad differential.

We changed the fluid, and cleaned the axle housing in the process. No noticeable shavings. The old oil, however was very black, and smelled horrible. After the fluid change, we notice the oil smell is still there, and the differential gets warm. I don’t want to change the differential if it is not needed. What are your thoughts?

Hi there,

I am not real sure what to think to be honest. I would assume the differential vent is working correctly since you say you can actually smell the gear oil. The rear end will be hot to the touch and more so if you are pulling a trailer or heavy load, so I am not 100% sure there is a problem there or not. Since you did not find any metal shavings in the housing…that is a GREAT sign.

I am not sure what kind of rear end you have, but I would bet GM has a special additive they want you to use, call the dealer parts department and ask about it. You might want to ask the parts and service guys if they know of anything weird or out of the ordinary with this rear end. I would NOT run out and change this rear end, I would ask about the additive and if GM is aware of a potential problem, but other than that I would keep my eye….and nose on it and monitor the situation.

What you don’t want is noise or fluid loss due to overheating, STOP if you experience those.

If you have a 4X4 or rear end shop in your area you might want to ask them if they have experienced this issue if you don’t get anywhere with the GM dealer. I rarely see rear end work in my shop and when I do I usually end up installing a used assembly from a junk yard due to price constraints.

Blessings,
Austin Davis