O2 Sensor Bias

[Bruce Plecan]

-----Original Message-----
From: Wen Yen Chan <chanwe at ecf.utoronto.ca>
To: diy_efi at efi332.eng.ohio-state.edu <diy_efi at efi332.eng.ohio-state.edu>
Date: Thursday, August 27, 1998 9:17 AM
Subject: RE: O2 Sensor Bias


Could this be as simple as a comparator, and a 555 for on after delay?.
Or could you share some more details?.
Cheers
Bruce


>Hello,
>
>A while back I built a circuit to change the O2 sensor readings so that I
>could make my engine run richer at idle. The circuit delayed the rising
>edge of O2 sensor's signal and left the falling edge alone. The circuit
>worked okay. To get around the computer's biasing, the output was coupled
>through a large resistor (200k worked) to simulate the output resistance
>of the sensor.
>
>l8tr
>Wen
>
>On Thu, 27 Aug 1998, Chris Conlon wrote:
>
>> Larry wrote:
>>
>> > A lot of guys play with fuel pressure to alter their WOT O2 readings. I
>> > understand that GM runs the stock motors a bit rich, and a lot of guys
>> > lean them out to get more power.
>> > ...
>> > shouldn't it be possible to just place a resistor inline with the O2
>> > sensor to "trick" the PCM that the O2 sensor is reading a higher
voltage
>> > than it really is, and thereby lean the mixture accordingly? Maybe I'm
>> > barking up the wrong tree here...but is this possible? How? Any
>> > problems?
>>
>> I don't know if any of this will help lean out WOT operation, but the
>> question about the resistor got me scratching my head some.
>>
>> You probably could use a resistor tied to a 1v bias to make the sensor
>> read a wee bit higher. (I'm pretty sure I've even seen flaky-looking
>> commercial products designed to do just this.) Picking the resistance
>> would probably be a pain. Also does an O2 sensor's output impedance
>> vary much, within the warmed-up temp range? This mod would probably
>> also mess up the 0.450v bias used to tell if the sensor is warmed up
>> or not, maybe leading to problems during warm up?
>>
>> The clever way requires you to have faith that the O2 sensor reading
>> is meaningful (not necessarily linear but repeatable) away from stoich.
>> Ok this isn't really true of an O2 sensor, but just pretend for a minute.
>> What you want, roughly, is an O2 sensor who's output crosses through the
>> 450mv range at say 16:1 rather than 14.7:1. Suppose your O2 sensor
>> reads 380mv at 16:1 (wild guess). Essentially you generate a 380mv
>> reference, and take the O2 sensor signal, and feed the 2 into a
>> comparator, or even an op-amp set up for maybe 20:1 gain. Divide and
>> buffer the output as needed for a 0-1v range, and feed that into the
>> ECM's input. Congrats, your ECM will now see cross counts at 16:1
>> instead of 14.7:1, and adjust the AF accordingly. (If it doesn't freak
>> out anyway.)
>>
>> Problems:
>> This also defeats the sensor-warmup detection via the 450mv bias.
>> You could add some more circuitry to get around that.
>>
>> The sensor voltage corresponding to 16:1 probably varies a good bit
>> with operating conditions, and the curve is steep to begin with. If
>> you had a wide-range O2 unit, well, that would be nice. :)
>>
>> How lean do you want to run? Detonation could be a real problem
>> depending on the engine.
>>
>> I realize this isn't a very useful solution to Larry's original
>> problem... but once the question of tricking the O2 sensor got
>> into my head I just had to think about it until I was satisfied.
>> Hopefully someone will find it useful or interesting.
>>
>>    Chris C.
>>
>
>