Burnt Rubber Smell Under Load


Hi All,

I've been trying to diagnose a strange smell on my 2003 Monty Limited. I usually smell it in the cabin with the drivers window down or the sunroof open, and am accelerating up a hill. Picture going on an uphill on ramp to a freeway. It also manifests itself when going up a long moderate incline that requires me to downshift the engine and run around +3k RPM. Another piece of data, about a month ago I was climbing up to Cero Gordo Mine from the Owens Valley, this is a steep road and I was running in low gear and about 2500RPM or so. When got to the mine I shut it down to take a tour and could smell it again when I got out. I looked under the truck to see if I could see anything obvious. The only thing I noticed was that the under coating on the frame and body areas around the exhaust pipes was "wet" looking. If I touched it, it left a burning hot sticky coating on my finger(think burning hot moly grease) type material.

Is it possible that the CATs are failing and causing the excessive heat, hot enough to liquify the under coating near the exhaust when running at high RPMs but low speeds?

I've ruled out a few other possibilities like rubbing tires etc, although that was my first thought as I only started noticing the smell after I put on my new (larger 265/75/R16 BFG's).

That's a lot of information to consider, but I'd be interested in hearing your opinions.

Some data about the truck: ~180k miles, with original CATs. Just passed CA emissions test, but I have seen intermittent O2 sensor codes, when when cleared don't come back for a while.
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I think you're on the right track with the undercoating being soft. Maybe drive it hard for a while then crawl underneath and see if that's where the smell is originating from. If that's the case then you're probably correct in thinking that your converter might be bad and getting too hot.

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