re: cats exhaust and fun

["estorhok" <estorhok@domain.elided>]

Dave Sisson asks about catalysts, and pointed out that (as I think I recall
from a few months ago) his exhaust smelled sulfery, etc.

You mentioned that the plugs were fouled, and though that the exhaust being
plugged caused it.  Its more of the chicken and the egg thing here.  Yes, a
plugged cat might cause the engine to run rich.  BUT, it is far more likely
that a rich running engine saturated the catalyst, slightly over heating it,
and causing it to produce H2S instead of H2O.  Some back of the napkin
chemistry here.  If you look on a periodic table of elements, you will note
the O and S are right on top of each other- sharing a valence bond level (i
think) at 2.  So if you run out of oxygen, which you do running rich, and
have plenty of Hydrogen around (like say HC, when its rich...), it will
start to encourage the HC + SO -> CO + H2S (forgive the lack of balance
here, I'm lazy).

Anyway, dave, if you want to "clean out" your car, its not a bad idea, BUT
you do want to run with a fresh O2 sensor (likely the cause of the rich
running in the first place).  Both an Italian tune up, and running steady
state- closed loop will clean the chamber out of much of the burnable
combustion deposits.

A reminder to those with catalysts- they fail in 3 ways- over temp, chemical
poison, or mechanical break.  This is a combination of over temp and
chemical poison.  Most older cars will over temp the cats just by their
rates of misfiring.

Hope this helps.
Eric Storhok
Ann Arbor, MI

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