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Re: O2 sensor diagnostics, duty cycle in %, long post

   Some additional info on O2 sensors and
   adjusting the frequency valve duty cycle.

I'll add my blathering to the fray...

   measuring is NOT constant and is actually cycling between TWO values as it responds
    to the O2 sensor voltage  cycling between the slightly rich ~0.9 volts and slightly 
   lean ~0.1 volts. If you disconnect the O2 sensor the ECU sets the duty cycle
    to a default value  around 50%.

Yeah, the "service manual" says something like 50% Plus/minus 6 or 8 %! Ha!
At least with the MAC-02-equipped UrQs, that value is 50% plus/minus zero
(my SunPro CP 7876 or whatever the number is reads 49.9%, every now and then
ticking to 50.0% rock solid). Any variation off of that indicates a problem

Dunno if this applies to the 5000/200/etc. turbos, but the MAC-02 has a neat
little self-test built in -- flick the WOT switch while at idle and the ECU
should go open-loop and hold the FQV at 50%; lots easier than disconnecting
the O2 sensor (unless of course you're testing the O2 sensor...)

Also, the ECU is not cycling between just "TWO" values, it's constantly
"servo"ing around the center point, alternately leaning/enrichening.

   After you adjust the mixture using the above procedure yada yada yada..........
   Ned Ritchie and others recommend just setting the duty cycle to around 42% (38 degrees dwell)
   with all O2 connected etc. I don't know if this recommended value of 42%  is 
   the "Average" value  between the two extremes or the minimum duty cycle seen.
   The above example on my car would indicate that the 42% adjustment recommendation
   is the minimum duty cycle seen and not the average. This gives you a mixture closer
   to the desired 1.2% CO upstream reading. The above readings assume my O2 sensor is
   responding correctly, others may want to comment on this.......

That's pretty much what I've adopted for my UrQ, I aim for a slightly rich
setting (fully warmed up/etc.) as indicated by the ECU leaning out the mix-
ture via driving the FQV at 44-45% (i.e., 40-48% typical readings; ya gotta
watch the meter for awhile and average out the readings; beware of the ra-
diator fan cycling on and off, affecting the idle, etc. -- always make your
readings in a consistent environment [e.g., radiator fan off]).

I have adopted a semi-annual Adjusting-Of-The-Mixture ritual, once late
spring/early summer for warmer/more humid air, and once late fall for the
colder/denser winter air (The MAC-02/UrQ is a pretty manual engine when
it comes to idle control; not to mention allowing for winter/summer gas
forumation changes!). The whole procedure takes 3 minutes; the hardest
part is finding that damned 3mm ball-end hex driver bit again . . .

Also note that the mixture adjust actually affects the full throttle range,
not just idle, although it is weighted (well, "cammed" is more accurate, I
guess) towards the low-end of the spectrum.

   Hope all this rambling helps

Yeah, what he said...