Proper Mixture.

[Donald Whisnant]

> From: Markus Strobl <eusmsrt at exu.ericsson.se>
> Date: Tue, 30 Jul 1996 13:24:07 -0500
> Subject: Re: Proper Mixture.
> 
> 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! :-)
> 

While we are talking on the subject of "block learns" and fuel pressures,
there is something I've been wondering about with this engine I've been
working with...  I've recently replaced the 85 3.8L V6 electronic
controlled carborated with a 94 4.3L V6 CMFI injection engine in my 85
Grand Prix -- And this 4.3L (because it is CMFI) requires about 60 PSI
of fuel pressure.  Obviously I had to put a fuel pump on the car (as
well as increase the size of the return line)...  When the tank starts
getting low (i.e. just entering the "yellow" zone on the guage), if I
hang a turn too hard, the fuel sloshes to one size and for just a split
second, the fuel pump sucks air...  Once this air makes it to the engine,
it causes a breif hesitation and loss of power -- the lower the fuel
level, the worse it is...  Once, it was so low (about 2+ gallons in
tank!) that it shut off totally when I made a turn at an intersection...
... My philosophy about this situation (and the reason I didn't have it
on the old carb system) was that the fuel bowl in the carb preventated
gas flow interruption --- plus I assume that the tanks on the fuel
injected systems have a well just below the pickup (with the pickup
extending into this well) to prevent this...

In a way, this is neat, as it is like a "reserve" --- when it starts
running out of gas, you definitely know it, but it won't leave you
stranded! -- as long as you don't take turns hard, you can make it to
a gas station ....

My question is what effect does this have on the PCM and/or the engine?
For a little instance (unless the poppet valve just doesn't open at all
because of the air) the engine (at least a cylinder or too) would be
running very lean -- which over time, can't be good on it .. Also, how
will the PCM deal with it on the block learn???

Donald Whisnant
dewhisna at ix.netcom.com