Propane type smell during hard tow???

[cbrock]

Two weeks ago I retired and sold my supercharged Z71 because the rearend wasn't able to handle the horsepower under the hood while towing. When i first got the truck it made a propane type burning smell during a tow up a long steep grade. Well, i got the 2500HD to eliminate these types of problems and sure as s**t, I smelled it again while towing up a very steep toll road near my house. Has anyone else experienced this problem? My trailer is only 8,000 lbs max too and it shouldn't make the smell. What i did on the other truck to eliminate this was get a larger rear diff cover and run Mobil 1 Synthetic gear oil. After that, it pretty much went away. I am probably going to do the same with this truck even though I didn't think I'd have to. What kind of rear diff fluid is everyone running and have any of you experienced this or heard of this....???

[jbplock]

I haven't experienced what you describe, but the smell could be vapor from the Hot Diff oil coming out the vent. I believe EP (extreme pressure) Diff oils have high sulfur content, which is what gives them that strong scent. Also be sure you drain and replace your DIFF oil after 500 miles of towing (per the owner's manual). Search the forum for more info on Rear Differentials and oil changes.

[ 01-23-2003: Message edited by: jbplock ]</p>

[AlanL]

My D-max (1850 miles) smells like its propane powered sometimes right out the tailpipe.

-Al

[Colorado Kid]

Apparently your truck is less than two weeks old and you're towing 8,000#...pehaps you should check out the break-in section in the book. It's been said here before (if you search) that you can severely overheat the diff if you don't break it in gently before towing, and wear in the towing pattern gently when you start towing. Tearing up a hill with 8,000# in tow does not meet the definition of gently, and your rear end problems may not be over.

I was probably too conservative, but here is what I did:
First trip in the truck was less than 4 miles at 35 MPH or less, driving gently. Let it cool down at least an hour.

2nd trip was about 10 miles at 45 MPH and less...still gentle. Let it sit again.

3rd trip was 40 miles...kept it under 50 MPH until 50 miles on ODO...still no full throttle, but made a couple easy runs up to 3,000 RPM (after 50 miles).

Continued to drive gently with occasional high RPMs till 500 miles, normal speeds but no cruise control until after 500 miles.

After 500 miles I played with the amazing power for a while, but without the trailer.

I first hooked the car hauler, with the '91 on it, at about 2200 miles, and towed it on flat ground for about 20 miles, speeds of 55 mph and below.

I then made several local trips with the 5th wheel, first 20 miles on the flat, then 45 miles on rolling terrain, then 70 miles, but still not in the hills. After 500 miles of towing I finally took it to Grandby (Colorado) which involved charging up I-70's long 7% grades at 68 MPH (Gotta love that D-max). When I got back from that trip I changed out the rear diff, using GM's priced like gold, grape scented wonder juice...which is the only product I could find that claimed to meet the specification cited in the manual (I'm an Engineer, and we're typically pretty anal about conforming to specifications...I'm not going to join the arguments about the relative merits of various gear lubes which don't claim to meet the spec., I'm just going to used what's called out.) The fluid in my diff did not seem scorched or particularly used up in any way...which was not the experience reported by some of the others here. Draining it did remove a fair amount of metalic grit however, so I'm glad I did it, and I wouldn't know how happy my rear end was if I hadn't checked. ;)

The highest temps a diff will ever see come in the first 500 miles of break-in, as the wear patterns are established. Once you get across that threshold (both "solo" and towing) there really is not much point in adding the finned covers and such...if you weren't gentle the damage is done and you've modified the metalugy of the gears, all the cooling in the world won't save them...if you were gentle you'll never get it hot enough to matter again.

[sdaver]

hey drive the truck like you stole it.........change the diff as specified.......the smell is probably what we all smell after changing the oil........you did not say how many miles on the truck.........there is a vent pipe on the crankcase that emits a fowl smell just after a oil change. It usually goes away after 3 to 4 hundred miles....... Its a great truck you want be disappointed. :D dave

[GMCTRUCK]

Two totally different trucks and both have a propane smell? Hmmmm...........What have you been eating? ;)

[cbrock]

Well, thanks for all the input. I didn't have time to change the diff fluid tonight, but i am definitely doing it tomorrow. I am ruling out the break-in period theory since since I far exceed 500 miles before my first tow and the tow was only a short distance. I have had all kinds of aftermarket rearends done with my other vehicles so I am very familiar with the break in procedures and although I tend to be pretty hard on my vehicles, I never cross that line breaking them in. I am putting in some Mobil 1 synthetic in tomorrow and will see what that does on my hill climbs. It has worked in the past. Actually, GM has a bulletin about this problem and some dealers deal with it at there discretion. (the good ones anyways) In my 2000 Z71, three local dealers didn't even have a clue of what it could be and it turned out that the dealer I bought it from knew exactly what I was talking about and showed me the bulletin from GM. GM did not have a fix or solution so that particular dealership was removing the factory stuff and adding Mobil 1 Synthetic. They told me that the mixture of the additives and stabilizers in the factory oil were mixing when heated and making the smell. Mobil 1 supposedly has the already in there and therefore not causing the smell when the rearend is heated up. It worked on the other truck so I have my fingers crossed. I was towing the same trailer with my Z71 so I should see better results with this truck. Thanks again everyone for all the valuable input and I will let you all know what happens.

[ItsMrBill]

cbrock, I hope they meant Mobil 1 Delvac.

[ram/tx]

cbrock----Your Z71 rearend required the additive in the pos-track, but our D/A rear end does not require the additive. It is a completely diffrent rear end and the clutches do not need the extra stuff. I have been running Mobile 1 synthetic in mine for 18,000 miles and have no smell and no problems what so ever with it.

[Just-Lee]

When the stock synthetic in the rear end overheats, it has a strong odor very much like the sulphur smell they add to natural gas and propane. I made the mistake of not breaking in my 99 properly and got the smell big time. When they pulled the cover at the dealer, the smell was horrid.

6 days and one new rear axle assembly and I was back on the road (it was toasted).

There used to be a service bulletin on this that said if the odor was reported, the rear-axle fluid should be changed.

[TraceF]

Maybe there's a propane leak on the hill you're going up.

tongue.gif

Sometimes I crack myself up.

Sorry.

[cbrock]

I changed the rear diff fluid on Friday and sure enough, the smell is gone completely. Man, the smell from the old stuff was terrible. I actually towed the same route also and pulled the same hill and no smell. The Mobil 1 worked. I must have just overheated the other stuff. Let's hope this smell doesn't come back.

[FisHn2DMax]

Yeah I've smelled some really bad propane in the cab while towing? However, I'm pretty sure it was coming from the passengers in the cab and not the truck. LoL...

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