Proper Mixture.

[Markus Strobl]

>  From what I've been able to gather, the O2 sensor found on modern cars is
> really only good for a narrow band of A/F ratios,  so narrow in fact that it
> is really only able to tell you if you are over or under the optimum.  Again
> from what I've gathered, this is used to adjust the injector pulse width
> slightly until the mix is a little too far the other way.  So, the A/F mix
> hovers close to stoich but never really stays on stoich too long.  

> Thomas Sparks

I wonder if this has changed in the last few years? when I tune my Z28,
I tune it by adjusting fuel pressure until I get about 890-920 mV from
the O2 sensors. This corresponds to about 13:1 A/F ratio. 

Interesting note about GM PCMs: If the fuel pressure (FP) is increased
above 40 PSI, the block learn for idle and cruise will start going to
less than 128 (effectively leaning the mixture). However WOT block learn
will *never* go under 128, and the WOT mixture will be rich. 

On the other hand if FP is decreased below 40PSI the block learn will
increase for idle and cruise to compensate but *also for WOT*, keeping
WOT O2 readings in the 890-920 mV range. 

Man, I'd love to see that source code! IF someone at delco is listening
send it to me and I'll be your life long friend! :-)


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Markus '96 Z28 w/ mods. Proof that OBDII is not the end of performance.