Message text written by "Randall Young"
>
> From: David Massey
>
> Dunno. Just that the new cars with computerized fuel
> management control
> the fuel/air ratio very very accurately and there are no
> problems with the
> cats.
Oh, I wouldn't say that. The entire mixture control thing depends on the
lambda or O2 sensor working correctly, and they definitely have a limited
lifetime. At the end of their lifetime, they start reading too lean. If
the computer doesn't detect that the O2 sensor is defective (and it's a
gradual process, somewhat hard to define precisely), then it can
overcorrect
the mixture too rich, which can cause an overheated converter. Having
computerized cars come in with red hot converters is a common sight in most
repair shops. This is a particular problem in cars with analog computers,
since they aren't very 'smart'.
Also, many cars with converters carry warnings not to hold them at fast
idle
too long, because of the potential for converter overheating. The
converter
naturally generates heat (and has to be hot to work), the design relies on
air flow under the car to keep other things cool.
Randall
<
The problem is agrivated at idle in carburetted cars due to the fact that
the air velocity in the intake manifold is not high enough to keep the
petrol atomized and the petrol will condense into larger than optimal
droplets that don't completely burn. This unburned fuel oxidises in the
converter which is an exothermic reaction creating heat.
The upshot is that the problem is worse at idle and running the engine
under load is less likely to create the problem. The other upshot is that
the port injected cars with injectors located very near the intake valves
have a reduced tendency to this precipitation problem. And the exhaust
gasses are cooler than the converter when it starts glowing red.
Dave
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