Rotten Eggs

When modern cars are running a little rich, they smell like rotten eggs (sulfur). How do they make this smell? I mean, is this the smell of a catalytic converter getting cooked to death? I know we have some chemists here, so I'm hoping for a brief explanation of the chemical process, if someone can provide it.

I richened my mixture in addition to unplugging my O2 sensor. Unplugging the sensor alone improved the car's performance. Richening the mixture also improved performance. but, in order to richen the mixture enough to eliminate ALL popping during deceleration, a subtle sulphur smell is created.

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'83 SC

Super,
I always thought the smell was more the cosmolyne coating burning off of the exhaust system.

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78 3.0L/3.2L CIS 911SC

Well, I'll admit I got a solid "D" in Intro to Physical Chemistry the first time I took the class back in 1975, so I'm still hoping to hear from someone who got a better grade. But I do believe that the sulphur smell is part of a chemical reaction taking place in the CAT. Then again, I believe in miracles too, so I'm waiting to hear a better explanation. Anyone?

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'83 SC

Your guess is right about the converter getting cooked. I used to own a Stealth RT/TT, and when it was due for it's major 60K maintenance I noticed the sulfur odor was increasing. After the tune-up was done (new sparks, wires, timing belt, etc. etc) the odor went away. I'm NOT an expert but you may be doing subtle damage to the converter, and if it clogs your performance will be greatly reduced.

The smell also can depend on the type of fuel. Mineral oil from certain parts of the world, e.g. South America, contain much mor Sulfur than other stuff, e.g. from Arabia.
This is not completely eliminated during petrol production.
By burning fuel, SOx is created which you smell.

The exact things I would have to look up as well
Jens

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[This message has been edited by Jens Wendorff (edited 05-23-2001).]

Hey Big S -
I missed this post somehow.

Your cat is fine, it just isn't working properly due to the decrease in available oxygen caused by the enrichment of your fuel system. The non-functioning O2 sensor is not correcting it.

Here is the chemistry. Some sulfur is present in all grades of gasoline. The amount varies. When the fuel is burned sulfur dioxide is produced. In an oxygen poor environment the SO2 is reduced by unburned hydrocarbons (while in the cat) to H2S. Hydrogen sulfide gas produces the rotten egg odor. It can be detected by smell at 1ppm levels.
When the A/F ratio is proper the NOx and SOx compounds are oxidized in the cat instead of reduced. The cat actually creates "acid rain" components when it is not working properly or in a low oxygen environment. If you can do it, I would get rid of the cat - and the smell.

Super...give your cat a rest, and put on a test pipe. You'll like the extra bit of power and smoothness. You can quickly bolt the cat back in for a smog test if you have to. Meow!

BTW... have you pulled your motor yet for the stud repair?

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Doug
'81 SC Coupe (aka: "Blue Bomber")
Canada West Region PCA
members.home.net/zielke/911SC.htm

Thanx, dudes! Got a followup question, Rarly (thanks for the explanation, it is what I wanted). When the CAT is doing the acid rain/rotten egg thing, is it plugging up, or heating up? I have seen CATs glow orange with heat.

Yes, Doug, I'd like to mount my CAT on my garage wall and may get a test pipe to do that, but I'd really like SSIs. I'm hoping my stock heat exchangers crumble upon removal like posts with dry rot, but I think they're in too good shape to necessitate replacement. And no, the engine is not out yet.



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'83 SC

The catalytic converter needs to be located close to the exhaust manafold due to the heat required for proper function. This is one reason why the thermal reactor cars run hot. A glowing cat could occur from an over lean condition, running at high RPMs for extended periods, or an oclusion. In your situation the cat is OK but not doing its job. Rich conditions usually result in a cooler running engine, for many reasons.

I appreciate this Rarly but I still have the same question. I'll guess at the answer: Oxydizing is what the CAT is supposed to do, and this reaction makes heat. In a low oxygen environment, less oxydization occurs and so less heat is created by the CAT.

Of course, lean mixtures create heat in the engine which is passed on to the CAT. Might this be true?

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'83 SC

The answer you seek was there, I just did not elaborate on it. The cat does not produce any significant heat from chemical reaction. Its sole function is to donate a "catalyst" for the oxidation reaction to progress. This reaction must take place under tremendous heat for maximum efficiency. The heat needed is found by placing the converter close to the exhaust manifold, and in many cases, by increasing the engines running temp through lean A/F ratios and cooling controls (245 degree thermostats and fan switches).
Oxygen is needed for oxidation to occur. Rich mixture = low oxygen, lean = high oxygen. When low oxygen levels occur the catalyst will switch to a reduction reaction using hydrocarbons instead of oxygen. The entire system will run cooler due to unburned fuel pulling off heat. Lean mixtures cause the engine to run hot.
The catalytic coverter doesn't care what reaction is going on inside of it. It is simply a pipe filled with metal balls. The temp is mainly governed by engine timing, cooling system controls, and A/F ratio.
Hope this helps out.

This does help, Rarly, and I thank you. I could probably understand chemistry (and everything else) better if I had not made such thorough use of it as a younger man.

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'83 SC