...Whats that funky smell?

In recent C4500 posts, I told of the oil leak following injector replacement that coated my truck in oil.

After the repair and half-ass underside degrease, I noticed a funky smell that I attributed to their degreaser burning off the exhaust.

However, a week and 1,000 miles later, the smell is stronger, and is rearward on the truck. It goes away in a few minutes, but you can smell it around the truck (downwind of course) for the first minute or two after stopping.

It smells to me like gear oil. The rear diff is not hot, just warm. The level is good, its Royal Purple synthetic with about 90,000 miles on it. Looked clean but clear in color the last oil / fluid change - I am wondering if its breaking down rapidly, or if I have gear issues out back.

No gear whine, no chatter around corners, just the funky gear lube smell as described. No visible leaks at pumpkin or axle ends, either, unless just a little is starting to get past the seals and is burning off the brake pads...now there is another theory...whaddya think?

Ever had a C4500 WITH A STINK? :)

Did the dealer who did the repair fill the engine with Rotella (or perhaps a non-common similar oil)? Ask 'em. Some oils are pretty stinky, and Rotella takes 1-2K before it doesn't stink.

Nope, no oil change when they fixed the leak, and its full of fresh Mobil Delvac synthetic. Also - the stink is definately at the rear of the truck, and its gear oil smell.

Since I was pulling a new heavy (22K# PJ tandem axle/dual wheel trailer, green treated lumber of ~16K#, total GCVW north of 32K#) - I am having the Royal Purple with nearly 100K on it drained today, and new synthetic 75/90 gear oil filled up. The tech is a friend of mine, he is going to look things over and see if there is any axle seal leak that can be seen from under the truck, or if it appears to have been venting excessively through the breather.

My current hypothesis is that the fluid is at the end of its lifespan, and breaking down under heavy loads, and the STINK is simply the outgassing from the breather vent when you stop first walk by the rear of the truck.

Tech reports no leaks, but said the fluid was in bad shape. Good news was minimal metal shavings on the plug.

My plan is to change the rear diff more frequently from here on out - probably at 50K. Considering the loads this truck tugs, fluid is cheap, cheap, cheap and the Eaton rear under this truck isn't bulletproof I have heard of failures.

Last year we ran into a smelly rear end :D on a 1/2 ton P/U that was running synthetic gear oil (factory fill). ANYWAY......... The owner had bought this truck new and never had any problems for 30,000 plus miles. She then bought a 24' toy hauler travel trailer and proceded to tow it home. She had a funny "PROPANE SMELL" from the rear of the vehicle. We chased and chased the smell. Finally found the bulletin from GM which suggests flushing the rear axle and adding fresh fluid. I am unable to locate the bulletin at this time. The flush fixed the odor smell.

The plot thickens...yesterday I got off the truck, and had wisps of smoke coming up off the right corner rear...:(

Knowing I am nearing the end of rear brake pad lifespan, I stopped on the way home and bought $795 worth pads and rotors at my local GM Medium Duty dealership - hoping I wouldn't need rotors which are very BIG, very HEAVY, and very EXPENSIVE!!! The pads are impressive - nearly twice the surface area of a 3500 series truck, and about 3/4" thick. The rear disc setup on the C4500 is impressive in its scale...

The truck's current brakes are all factory stock at 118K...all heavy duty nationwide towing miles - so I am very satisfied with OEM performace thus far. The factory exhaust brake certainly helps to extend the lifespan, I would estimate 50% or more of braking happens between the exhaust brake and the Allison grade braking (tow/haul) which is constant in the medium duty line - not 'user optional' like the gear shift button on 2500 and the 3500 series.

Today the truck was back up on the rack - good news was no brake rotor damage, and according to the tech, 1/8 to 1/4 pad is still remaining! The tech reported smelling gear oil at the right rear corner - but didn't have time/parts today to start tearing further into it since the truck was needed back by me for a trip to Wisconsin tommorrow. We decided to button things up leaving the current brake hardware in place to avoid oil soaking the brand new pads if the seal leak were to get really bad quickly.

As of now, everything easily visible still appears dry, but there is definately gear oil getting behind the backside of hot rotor - initally creating the smell, and then the visible smoke. I haven't seen it apart, but apparently the seal area must be very hard to see without some further disassembly of the wheels and hub.

Next week we'll get back into the rear, and replace seals on both sides to resolve the problem. We'll install the new pads then, and return about $500 worth of unneeded rotors...:)

Totally forgot about the obvious!!!! Wheel bearing seal................ Can't think of them all. :D

Just an FYI. Found the bulletin for the funky smell with no leaks detected........................... :D





PIP4027A: Propane Sulfur Or Foul Odor From The Rear Axle - keywords differential fluid vent - (Apr 8, 2008)


Subject: Propane, Sulfur or Foul Odor from the Rear Axle


Models: 2007-2008 Cadillac Escalade, Escalade ESV, Escalade EXT

2007-2008 Chevrolet Avalanche, Silverado, Suburban, Tahoe

2007-2008 GMC Sierra, Yukon, Yukon XL




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This PI was superseded to update model years. Please discard PIP4027.


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The following diagnosis might be helpful if the vehicle exhibits the symptom(s) described in this PI.

Condition/Concern:
Odor or repeat odor from the rear axle with no other noise symptoms.

Recommendation/Instructions:
Drain and disassemble the rear axle by removing the gear case and axle shafts.
Clean the rear axle and gear case completely with a suitable solvent make sure the housing is clean and dry prior to reassembly.
Replace the rear axle vent hose and cap.
Relocate the vent to either near the spare tire or evap canister area.
Cut the new vent hose approximately 6 inches from the vent cap end and install filter part number 5651682, secure the filter with clamps supplied.
Fill the axle with the appropriate fluid listed in SI.
Please follow this diagnostic or repair process thoroughly and complete each step. If the condition exhibited is resolved without completing every step, the remaining steps do not need to be performed.