I have a 3.2l porsche engine that is pre obd. I am changing the O2 sensor and want to evaluate the the idle mixture. I am working with the assumption it is rich because the idle hunts 600 to 1500 a symptom known in these engines to be associated with a rich idle mix.
I pico scoped the sensor with the heater powered not connected to the ecu.
The oscillating signal we are familiar with is not a function of the sensor but of the ecu's method of deriving stoich so we don't see that unhooked from the ecu.
The scope trace is a mostly d/c line with some oscillating noise. The scale is the usual .1 to .9 volts.
There is an idea on line (a couple of tuning sites) that the sensor can't give good information at idle because of insufficient exhaust flow. I find this a bit dodgy. My sensor is in a bung in the collector no cat. The exhaust flow seems plenty to wash the sensor with exhaust gas at idle.
The trace at warm idle is .8 volts, suggesting rich mix, however adjusting the mixture screw doesn't bring the trace up or down.
Can the O2 sensor be used in this way, and has anyone tried this? I am aware that a broadband sensor in this bung would give a more linear voltage change with mix, but the idea that the data is useless at idle deters me fom getting one until I hear some opinions from the group.
An O2 sensor may not report accurately at idle because it doesn't reach/maintain operating temperature. So, it's not so much a matter of exhaust volume, it boils down(no pun intended) to exhaust temperature being low at idle. Not only OBD 1 but also some OBD 2 vehicles will default to open loop at idle. Sensor/heater design are critical for this decision.
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Last edit: 3 years 10 months ago by juergen.scholl.
Thanks for thinking about my issue. The nb O2 sensor has heater wires and I hooked them to a spare battery. I measured the current of the heaters on my scope so I knew they were active. It heats up in a minute or two at most.
Also visavi the open loop idle mix my stock prehistoric motronic fuel injection system also idles open loop until it warms up, then if the O2 sensor is active it goes closed with a hot engine.
The system uses a map at
cold idlel so I wonder why the nb reads .8v which seems very rich.
In the op I read "with the heater powered not connected to the ECU". I took this as not heated whike you did your test......
This early Motronic system may feature an idle enrichment strategy, depending on operation conditions. On cold idle it will definitely enrichen the mixture.
An expert is someone who knows each time more on each time less, until he finally knows absolutely everything about absolutely nothing.
Last edit: 3 years 10 months ago by juergen.scholl.
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