Q A reader asked what causes the stinky smell on Interstate 680 over the Sunol Grade. Come on, Gary. She was pulling your leg, I am sure.

Tom Boylan

A Ha! Janice Carr of Los Altos inquired about the stink on I-680 and other high-speed roads when traveling up or down a hill, saying she apologized "for such a trivial topic." Trivial? Dozens responded, from drivers to auto experts.

Q Regarding the complaint about the stinky smell on 680 over the Sunol Grade: Aren't there a bunch of organic farms there?

Jeff Moller

Livermore

A There are, but that's not the likely cause.

Q I sometimes I notice a bad smell on Interstate 580 through the Altamont Pass. ... Sometimes coming down I-80 from Reno I will detect an odor. ... Could this smell be burned rubber from tires off big rigs? ... I live in Concord and have worked in Fremont for 26 years and I have smelled that stink several times on the 680 grade. That is a sulfur smell. Sometimes it is pretty bad; other times it is not there.

Bobbie Gidish, Tom Flack, Stan Klosinski and more

A On to the search for answers!


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Q Regarding Ms. Carr's "road stink": The mention of the Sunol Grade was the giveaway. I have traveled that route since the early '90s and often, if not regularly, notice the smell of ozone near the crest. It is not as bad or as often as 20 years ago, when you could see the blanket of brown smog hovering over the valley. I'm willing to bet ozone is a major component, if not the actual source, of "road stink."

Jerry Straks

Fremont

A Maybe we should peek under the hood.

Q Responding to "road stink" -- I bet what she is referring to is actually the emissions from a catalytic converter. When a catalytic converter starts to die, it loses its ability to process hydrogen sulfide (otherwise know as the rotten egg smell).

This smell is especially prevalent when a vehicle is under load, e.g., traveling up a long, uphill grade like 680 over the Sunol Grade. And more than likely it is from a vehicle in front of her, not her own vehicle.

Ken Buhagiar

South San Francisco

A Ken is onto something.

Q I assume she is not just talking about the usual "Milpitas stink," so I think the thousands of catalytic converters going up the hill are the likely cause of wrinkled noses.

Since gasoline contains some sulfur, it is present in the exhaust. Catalytic converters (aka cats) don't really tackle sulfur per se. The modern "three-way" cat is designed to reduce nitrogen oxide, oxidize carbon monoxide and oxidize any unburned fuel that comes out of the engine.

What happens to the sulfur? Most goes out the exhaust, but some collects as a deposit on surfaces inside the cat during low-temperature operation. When you climb a hill, the engine is working harder and you get more of "everything" in the exhaust, making the cat go from 1,200-1,300 degrees to as high as 1,800 degrees.

The higher temperatures release the accumulated sulfur, which then combines with oxygen to form hydrogen dioxide (which smells like a struck match). This is perfectly normal.

Alex-the-Auto-Man

A What about going downhill?

Q The catalytic converter cools and the exhaust odors return to normal. That's when you can start getting overheating brake smells, but these are less common today with modern brake designs that cool better than in the past.

Alex-the-Auto-Man

A And for the final word. ...

Q I have been an automotive technician for 30 years and you can see, hear and smell that some cars are struggling to keep up with the high speeds of everyone else. This means that their drivers are flooring the gas pedal, which sends gobs of unburned gas into the catalytic converters, which then gets converted into hydrogen sulfide -- the smell of rotten eggs.

It has been more common recently. Because of the economy people are now keeping their cars longer. Since all engines gradually lose power in relation to the mileage on them, you see vehicles with 200,000 miles having to work twice as hard as one with 50,000.

Gordon-the-Auto-Tech-Guy

A Mrs. Roadshow, roll up those windows on our next trip to Davis.

Follow Gary Richards at twitter.com/mrroadshow, look for him at facebook.com/mr.roadshow or contact him at mrroadshow@mercurynews.com or 408-920-5335.