If the computer does not see 500mV it will keep on changing the mixture until it gets to 500mV. If it cant get there it will go into limp home mode.
For example lets say HEGO sensor voltages are:
14.7:1 500mV <--- lambda 1
The problem is that the HEGO has a very narrow band of opperation approx 14.5:1 to 15:1 afr.
lets say we want about 15.5:1 for fuel economy and minimal damage to the cat. The Black line is stock and the red line is what we want the computer to see.
Ideal = Red line
going from black to red line
there is no linear relationship here. A resistor, linear amplifier or constant voltage will not work.
The problem with using a digital microcontroller interceptor and narrow band HEGO is that the voltage differences are so small that a change in temperature will change the tune significantly.
I dont even think a "aftermarket chip" would change the closed loop afr.
Maybe someone with a uni chip could test this for us?
The only option that i can see working is using a wideband HEGO an amplifier and some switching MOSFETs. This will cost about $100
above is a current vs afr curve for a wideband sensor.
Now for my plan. i will try and describe it to the best of my ability.
get 3 voltage sources 50mV, 500mV and 900mV.
The aplifier will amplify the HEGO output to usable voltages.
50mV is always connected to the ECU.
500mV is switched off at >0.1mA equivelent 16:1
900mV is switched off at >0.05mA equivelent 15.5:1
Then use some resistors and caps to smooth the switching so the computer doesnt have a heart attack.
is a really good wideband AFR meter for only $355 if you DIY. this would be the best way to do it as the AFR meter has a linear voltage out ready to drive the MOSFETS.
This is a handy little project.
Closed Loop Monitor