I would agree, sounds like the CAT is failing.

 

Welcome to the site,I would run it in to the dealer I'm sure something is not right, I have heard that the flapper valve in the resonater could be sticking in the 4cyl. mode?, a friend of mine took his in that was his problem, maybe yours is sticking?, did you change brand/grade of fuel? just saying...

 

Run some SeaFoam through it and see if that helps.

 

Thanks all, I just installed by RANGE AFM Disable device this AM and noticed the hot smell is still there so that eliminates the flapper in 4cyl mode. Dropping it off to get my frame rust dealt with next week so I'll have them look at it then.

 

Took truck to my dealer and explained issue, they said there we no codes and they couldn't find anything wrong. I asked how to explain the hot CAT smell and the much lower gas millage and they just shrugged their shoulders. I ran one can of Sea Foam through, I guess I'll try another can and see if anything improves. After more driving I can say (without AFM lockout installed) I've gone from 18.5 to 16.5 around town and from 21.5 to 18.3 highway. Anyone ever experience worse millage with the 'Winter Blend' gas? I know the stations around here started to swap to winter blend recently.

 

Winter blend will definitely yield lower fuel mileage >>> and with the cooler temps it's taking a bit longer for the engine to get up to operating temp. ~~~

 

Update - Yesterday I loaded up 420 lbs of counterweight and connected my snow plow. I then ran an errand and noticed the hot smell stronger than ever (in cab when I stopped). The CAT is definitely burning out but the real issue for me is why? Even if it's replaced (eventually) under warranty it's just going to burn out again if they don't find a cause. Pretty sad to have a CAT burning out at 17k.

 

How do you know it is the CAT burning up? I always see drops in MPG with the winter blend, normally around 1-2 MPG

 

I can't say 100% but this all started when I first hauled my camper and got a strong sulfur smell from exhaust. After that the consistent hot smell started and it seems to be strongest between cab / bed on passenger side (where CAT is).

 

I have seen a lot of 4X4's puke fluid from the Tcase when towing or under extreme conditions, I would look at the vent on the tcase to see if it has signs of leakage.

 

Well, I would try a couple simple things. First, I would get down under 1/4 tank and then fill up with some 91 or higher octane gas, preferably without any ethanol. Then see if that changes anything. The next thing I would do when I start smelling the burning smell again is leave the truck running and look under the truck to see if the Cat is glowing. If it is, it is then it is probably clogged and the cause of the burning smell. Even if it isn't glowing but looking and smelling very hot, then it could still be the issue, just not to the point of being fully clogged.

Those are a couple of easy and inexpensive things to check out.

 

One of the by products produced by a catalytic converter is sulfuric acid. If your economy is dropping that means that you are putting more un-burned fuel through the cat and producing more by products hence the sulfer smell. Do you have a scan tool or know someone who does? The first thing I would check is that everything is within acceptable parameters. I do not know about the new systems as I have been out of the business for many years but I used to see things out of spec but not out far enough to set a code. The combination of low economy and sulfur smell says to me that the cat is seeing too much un-burned fuel.

 

And higher octane gasoline burns slower which could increase unburnt fuel in the exhaust mixture making the problem worse not better. Why do people suggest more octane as a cure for random problems, these trucks with the 5.3 use 87 octane no more is needed or required.

 

One possibility and the OP never mentioned it but when switching from E85 to E10 you need to run the tank to 1/4 or lower and after refueling with the first tank of E10 you can't shut the engine off after the fill up, the reason is the ECM does a fuel content sniff test to see what fuel has been added, it does this when it sees the fuel level changed from the last run test. If the ECM sees E85 in the tank it will adjust the mixture parameters which could be way to rich if running E10. You should also do this procedure when switching to E85 from E10

 

One possibility and the OP never mentioned it but when switching from E85 to E10 you need to run the tank to 1/4 or lower and after refueling with the first tank of E10 you can't shut the engine off after the fill up, the reason is the ECM does a fuel content sniff test to see what fuel has been added, it does this when it sees the fuel level changed from the last run test. If the ECM sees E85 in the tank it will adjust the mixture parameters which could be way to rich if running E10. You should also do this procedure when switching to E85 from E10

Thanks for the feedback everyone, I'm a stickler for not letting my truck run below 1/4 of a tank so this may explain it. So run it below 1/4 tank, fill it with ..?? (does Octane matter?) and how long do I need to keep it running after that fill up for the stiff test? 10-15 minutes? Also, I got under the other day and didn't see any fluid issues. If this sniff process doesn't work I'll crawl under and look for the glowing CAT.

 

You only need to worry if the truck is a flex fuel vehicle and only when switching between E10 and E85, but it does the sniff on every refuel and it will run the test between 2 and 15 minutes from start up. Octane doesn't matter just ethanol percentage content.

 

Just for the heck of it,, have you had it to another dealer to have it checked out? seems to me a fairly new fairly low mile truck they will find out what's wrong before crawling under, changing fuel, AFM disable etc. or other things you have done or tried, just sayin', my .02 worth

 

i have a 2014 5.3 with 20k miles .the cat smell on mine started around 14000 miles . i took it to the dealer and they said the same thing. i don't have time to waste so i kept the truck and did some diagnosis on my own. out here in california this top tier gas crap really made it smell bad especially when driving hard and coming to a stop. that rotten egg sulpher smell totally surrounded the truck. i started using 87 octane united oil, which is similar to arco brand. and no b.s. the smell is totally gone, for months now i haven't smelled it. i'm really interested if you find anything with your truck. i too have the afm range tool and i modified the exhaust flap to stay open all the time.

 

wildchevy is right higher octane is not the answer. probably higher ethanol is better

 

Heres a thought...

I do a lot of exhaust work and I cut mufflers open for fun..

A lot of these new mufflers have insulation in them.. Fords I know for sure...

So here is where the smell MIGHT be coming from.... The insulation is in a plastic bag... No joke. They have it pre-wrapped and stuff it where it goes in the muffler.. Maybe for ease of manufacturing or safety of workers??

After a few hundred miles, the plastic will start to stink. With the combo of the nasty insulation that looks like Wookie fur and the burning bag over the next 1000-2000k.... It smells rotten eggish.

Now.

GM has changed their muffler design over to something a lot like what Ford is doing. And the rear resonator on the tail.. It could even be that... It would take much longer to crisp up those fibres and get the bag burning...

Sound like hocus pocus, but think about it. I know this is something that happens to some new vehicles..

 

There is nothing wrong with your truck. With the cooler weather (good for a 10-20% drop in fuel economy) and winter blend fuel (good for a 2% drop in fuel economy), the fuel economy drop is within reason. The smell you smell is the coating on the exhaust. It happens to ever single one of these after the first true hard use (pulling a camper, towing heavy, etc), it starts to burn the coating, some of the undercoating can also drip onto the exhaust, resulting a long-lasting odor.

There is no plastic in the exhaust, so don't worry about that, I have had a bunch of these mufflers pulled apart on these very vehicles.

I don't believe fuel is causing your smell either, since you indicate it's not the exhaust coming out of the tailpipe that has the odor, but rather the exhaust piping itself, near the cat.

Provided you have nothing leaking, getting onto the exhaust....chill out and enjoy your truck. It doesn't need a cat or such,

 

So I went below 1/4 tank, filled it and drove it for a while, no change. I've actually had 2 other people walk by my truck after driving it recently and say 'man, your truck smells like something is burning'. I've tried 3 different brands / octane levels too. I'll be going in for an oil change in a couple of weeks so I'll press the issue with the dealer at that time.

 

I would notify gm customer care then ask them which dealership you should take the truck to. don't mention the fuel economy, once you mention fuel economy they'll assume you're full of it and just some complainer, just the issue with the smell. trust me it's not normal and it's not made up. Im a tech for a german car line and we blow out customers concerns all the time, but if you get the factory involved they'll assign it to a tech that gives a s***. I guarantee my 14 has the same smell, its not all the time , and once I switched to regular gas it totally stopped happening. please keep us up on any results. thanks .

 

I'm going to go with RagsMatt's theory, for about the past 10 days the smell dissipated, the only time I noticed it is when I drove 20 miles with my plow on. It typically runs hotter then so it wasn't surprising.