Re: Multi-function sensor woes....

[walter@quattro.corp.inmet.com (Walter Meares)]

> 
> I'm not real clear on this but here goes..... I believe its called 
> unmetered air, The cars computer is very particular about metering
> the air in the engine. 

Not even the computer.  The K-Jetronic injection used in most (maybe all?)
Audis uses mechanical means (the fuel distributor) to provide the basic 
fuel metering.  Whatever air has passed through the fuel distributor is the 
"measured" air, and the fuel distributor has implicitly metered in a correct
amount of fuel.  Any unmetered air entering the plenums or vacuum taps after 
the fuel distributor will be unaccounted for and cause a lean air/fuel mixture.

The computer's role is to fine tune the mixture to hold it near stoich for
best catalytic converter performance.  The computer does this by adjusting 
fuel pressures in the system based on feedback from the oxygen sensor.  Which 
pressures it adjusts and how those pressures affect mixture strength depends on
which version of K-Jetronic the car uses; there are a couple of good books on 
this subject if you want the gory details.
 
Note that PCV blowby does not affect mixture strength because it consists 
either of unburned, but metered air/fuel mix (hence at the correct mixture) or
exhaust gas, which is inert (no air or fuel in it).

> I'm sure there are some other people on the net who are much more 
> knowledgable than I on this subject,  maybe they'll post a better
> reply :-)
>

Others probably know more, this was a quick summary. EFI is nice for lots of
reasons, but the elegant simplicity of the SU sidedraft carbs on my old z-car 
are hard to beat.  (but no where near as clean-running!)

 
> Mike (smooth running 89 100 again.....) L.
> 

Walter

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