weird o2 sensor readings? (mc'd urq)

[Nate Stuart]

> so, here's where the weirdness started... the display was showing
> about .400-.600v at idle, and would drop quickly under load, to
> almost zero....  so, i pulled over and turned the mixture screw about
> 1/2 turn clockwise (i know that's alot)... enough to cause the
> frequency valve to shut off at idle... got back in the car, and at
> idle, itwas reading about .800-.900v... ok... SEEMS closer... but
> assoon as i got on the gas, it would drop...

You cannot use a multimeter to tune a closed loop O2 system. The O2 sensor
voltage does not stay at a steady point, instead it fluctuates back and
forth very quickly from lean to rich readings, as the ECU adjusts the
mixture. I have yet to see a multimeter that refreshes/samples fast enough
to even come close to properly displaying an O2 sensor output, especially
under load/high RPM, where the cycle rate is very fast. Do not tune the
engine based on the multimeter readings. You're not going to be able to
ever get the 'correct' O2 reading this way, as the ECU will continually be
adjusting the mixture anyway.

> but here's the strange thing.  the car had a LOT more power, and the
> reading dropped BELOW 0v, into 'negative' voltage, about -.02 or
> so....  and finally, when i turned off the car, the voltage reading
> stayed there, and was around .550v

Yup, things like that will happen, it's usually because the meter is not
capable of measuring the fluctuating signal correctly.

> but, like i said, the power is closer to where it is supposed to be,
> it isnt using coolant, the compression is almost exactly the same
> across all 5 cylinders, the o2 sensor is brand new, the idle/wot
> switch is new, the temp sender for the isv is new, the michelin man
> hose is new, i checked and rechecked the intake hosing, inspected the
> vacuum lines.....nothing....

It may have been that you were running lean before, for some reason, so
your adjustments may have helped, but they are probably not where they
should be. Wait until you get that real A/F ratio gauge. At the very least
that will have a fast enough display to show you roughly where your
mixture lies. Also, double check the Bentley for the proper baseline
setting of the CIS system, it's really a lot simpler than you are making
it out to be. If I recall correctly you just need to measure the OXS
valve's duty cycle at warm idle, and turn the mixture screw until the duty
cycle falls within a specified range. It's been a while since I've ever
mucked with this (yay for programmable efi ;), so I may be way off of the
procedure, but it is in the Bentley. If you need I can dig it up sometime.

> the one thing i dont have on hand is a fuel pressure gauge, but im
> gonna order one, since it seems like it's a good idea to have anyway,
> esp w/ the cis....
>
> anyone have any thoughts?

Yeah, that definitely doesn't hurt to have with a CIS car, as everything
is based off of the system pressure.

Take care,
-Nate
'89 90tq
www.newtsplace.com/90tq