Re: Setting fuel mixture using dwell meter....

[Dan Bubb <jdbubb@ix.netcom.com>]

To start at the start, the Frequency valve controls fuel pressure in the
chambers below the diaphragms under the fuel delivery ports. Reducing
the pressure causes the diaphragms to flex down slightly opening the
flow area into the fuel delivery tubes that lead to the injectors and
richening the mixture slightly. The Frequency Valve duty cycle changes
based on input from the O2 sensor. So, if the mechanical setting (the
3mm hex wrench set screw) is set too rich the duty cycle will fluctuate
around a lower % on as the control computer brings the mixture back to
stoichiometric. As the mixture is adjusted leaner the duty cycle will
fluctuate around a higher value.
So, as long as the engine is running in closed loop changing the mixture
setting (within reason) will have minimal effect because the computer
keeps changing the duty cycle for a stoichiometric mixture.
But, the engine isn't always in closed loop. Start, cold running and
full throttle conditions all go to a fixed duty cycle. (80% and 60% if I
recall correctly) So, under these conditions the computer is not
compensating and the mixture will be off if the mechanical setting is
not correct.
The engine should be set so the meter fluctuates evenly around 50% or 45
deg.
As a side note, there is a way to adjust the full throttle duty cycle to
richen the mixture if anybody feels that's necessary. I'll have to look
this up.
Now that I'm comfortable with my scanner, I have stuff I could scan that
would help to understand my above gobbledygook presuming it was clear as
mud.
Dan


Shawn C Meze wrote:
> 
> On Sun, 19 Aug 2001 21:09:05 -0500 "Brian Wagner"
> <scirocco_88_16v@hotmail.com> writes:
> >Should fluctuate 23 to 59 degrees on the dwell meter
> >
> >45 degrees = 50% duty cyle
> >
> >1% duty is the equivalent to .9 in dwell
> 
> OK so all that means is the Frequency valve Duty Cycle works within spec.
> Nothing to do with fuel mixture?
> 
> Shawn Méze
> 86' Jetta GLi 8V       84' Scirocco 8V           88' Corvette -SS 30-
> 82' Scirocco GTi -FSP 54-     79' "Project FSP Scirocco"
> The Fastest, Quickest, Cleanest and best looking Scirocco in all of San
> Diego!
> http://www.Geocities.com/MotorCity/Speedway/1308/index.htm
> 
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