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Discussion Starter · #1 ·
I've a 2016 iTouring automatic with 5,500 miles. I was entering an expressway and merged with traffic from a standstill. I pretty much depressed the gas pedal to the floor for about 4 seconds. After a few seconds, I smelled a burning odor (like burnt tires). Though I'm not 100% it was my car, should this be of concern, especially for a new-ish car?
 

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It's hydrogen sulfide or H2S. It does not exist in the fuel nor oil, which is why you can't smell it at the gas station. It does, however, come from the gasoline. Almost all gasoline contains some amount of organic sulfur compounds with sulfur levels ranging from 30 to 100 parts per million, approximately.
As the fuel burns, the organic sulfur compounds break down into simpler compounds. If the engine is running lean (more air than is needed to burn the fuel), the sulfur is likely to be SO2, or sulfur dioxide. The cat can change this into SO3 (sulfur trioxide), which then reacts with the catalyst to form sulfates on the catalyst. Over several minutes there is a build up of sulfates. If the engine suddenly starts to run rich (not enough air to burn all the fuel), such as when you stop at a red light, climb a steep hill, drive spiritedly, or brake hard to slow down, the chemistry changes. Now the sulfates are unstable and they react to form hydrogen sulfide, which is expelled from the exhaust pipe all at once. The high concentration is the reason for the bad smell! This effect is a common problem with just about any new vehicle of ANY BRAND/MAKE because the catalyst is working at a very high efficiency. However, as the catalyst ages, the activity drops. Fortunately for us, one of the first reactions to be affected is the formation of hydrogen sulfide. So as the catalyst gets older, the smell will go away. It should also be noted that many catalysts never smell bad. One reason might be because the vehicle doesn't run lean for much of the time, or perhaps the catalyst has an extra component that reduces the production of hydrogen sulfide.
 

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Discussion Starter · #3 ·
Thank you very much for the detailed response! So it's safe to say the burning odor shouldn't be a cause for concern? At how roughly how many miles should the smell go away (if driving spiritedly)?
 

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For those with new 3's that have come over from Japan, you will especially notice a distinct, different order at the very beginning of your ownership. That too is normal, part of the protective coating they put on it as part of its trans-oceanic voyage. It will start to become more and more infrequent over time, but even now with 900 miles on mine, now two months since it left the dealership, I can still occaisionally smell it for a few seconds. [That however is different from the beginning of our ownership when we smelled it a fair amount of the time.]
 
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