At 10:16 PM 4/11/2010, Jake de Villiers wrote:
>That's a charming anecdote David, and I'm sure it brings back
>peasant memories, but it still doesn't explain how hydrocarbon
>emissions rise when running without an oxygen sensor WITHOUT
>increasing fuel consumption.

It's because the CAT requires that the mixture run at stoichiometric
ratio (14.7:1) in order for the raw exhaust to have the exact
proportions of hydrocarbons, CO etc so that the three stages of the
CAT will each supply the right proportions of reaction products for
the next stage to work with.  If the mixture goes too rich you get
hydrocarbons; if it's too lean you get nitrogen oxides.

But stoich is in between the best mixture for economy and the best
mixture for power.  The engine will run perfectly well with much more
variation in mixture than the CAT will operate correctly with.

If you disconnect the O2 sensor the ECU will know it and control the
mixture according to its internal maps.  As I said, this is the
normal operating mode in any location where leaded gasoline is used,
because neither sensor nor CAT will survive operation with leaded gas.

If the motor is not working correctly the O2 sensor may very well
compensate for the problem, in which case the motor won't run well
with the sensor disconnected.  If your engine is going full rich
without the sensor, then there's something very wrong with the engine setup.

Yours,
David