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Tricking the O2 sensor with resistors

isaac

isaac

The Isaac
Joined
Nov 13, 2002
Location
Hurricane River Street, FL
Nick gave me this idea yesterday to make a car get better fuel economy: Tricking the O2 sensor.

Basically you run 12v through a resistor/potentiometer to step down the voltage, swapping resistors until it's about .05-.1 volts and you run that extra voltage to the signal wire for the sensor, thereby artificially adding a bit of rich signal to the O2 sensor and making the computer continually run the car slightly leaner.

Anybody ever done this with success?
 
I would actually bet this hampers fuel economy. You're going to be using more throttle to compensate for a doggy bad mixture. Plus, she's gonna be runnin' hot. Good way to make a turbo or manifold glow (if it's leaned out enough).

Jim
 
rdp001 said:
if you screw up on this one it will be burnt valves!!
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doubtful.

I would actually bet this hampers fuel economy. You're going to be using more throttle to compensate for a doggy bad mixture. Plus, she's gonna be runnin' hot. Good way to make a turbo or manifold glow (if it's leaned out enough).

Jim
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maybe. depends on the total amount of lean-ness.
 
Sorry I forgot to add the part about me not being stupid enough to run the car at 16:1 all the time. Yeah.

That said, lets say I wasn't stupid, and instead wanted to, as I said, bump the AFR from a 14.7 stoich center point to about 14.9-15.2. Eh? Eh?

This is to correct a car that's still running a bit rich with a new O2 sensor btw.
 
isaac said:
Sorry I forgot to add the part about me not being stupid enough to run the car at 16:1 all the time. Yeah.

That said, lets say I wasn't stupid, and instead wanted to, as I said, bump the AFR from a 14.7 stoich center point to about 14.9-15.2. Eh? Eh?

This is to correct a car that's still running a bit rich with a new O2 sensor btw.
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Did you change only the O2 unit and see an immediate drop in mpg? I ask because I recently changed my O2 and ECU and immediately lost about 5mpg hiway on a 240 NA that I've been driving for over 10 years. The mpg didn't improve with an ECU change so pretty sure its the O2 unit. Extremely annoying. The O2 unit I used was a Bosch "universal" for Volvo according to Ebay. Don't know the part number. Makes me wonder what the variance between O2 units is, or if the one I got really wasn't the appropriate part.

If decreasing signal voltage is interpreted as increased exhaust richness by the ECU then the resistor trick would be easier and tunable. Anyone know?

second edit: I see from the Bently manual that it's the other way 'round, so not as convenient to calibrate it with an in-line resistor.
 
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I put a wide band on my LH2.2 240 and it read close to 14.7 at cruise, but often had AFRs in the 14-15 range while accelerating. Sometimes I'd even see mid 15s while at WOT. If I made it a little leaner over the range, I'd probably get better fuel mileage, but...I don't think I would want ~16:1 at WOT.
 
slowmotion said:
Pretty much that's what I mean. Different part, same result (lean or rich depending on voltage fluctuations).... but 2.4 will learn them out right?
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Not really the same result, no. When you install resistors in the MAF line, the O2 sensor will adjust at cruise to a certain extent but at WOT its totally trusting the MAF.

In this case its going to be ignoring your modifaction at WOT and trusting it at cruise.

edit: i ****ing HOPE it ignores the narrowband at WOT
 
It will probably continue to adapt to the limit because your "signal" is continually rich or lean. So it will gradually get too rich or lean unless you have it hooked to a rheostat that you are continually adjusting. If it is lh 2.4 it will set a code for the o2 sensor signal being faulty/slow response and make the car run poorly in "failsafe" mode.

edit: I have seen this done on later obd2 cars to trick the computer when a test pipe is fit, but only for the rear o2 sensor which gives a constant voltage when the cat is functioning correctly (usually around .4 volts).
 
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wennstroma said:
It will probably continue to adapt to the limit because your "signal" is continually rich or lean. So it will gradually get too rich or lean unless you have it hooked to a rheostat that you are continually adjusting. If it is lh 2.4 it will set a code for the o2 sensor signal being faulty/slow response and make the car run poorly in "failsafe" mode.

edit: I have seen this done on later obd2 cars to trick the computer when a test pipe is fit, but only for the rear o2 sensor which gives a constant voltage when the cat is functioning correctly (usually around .4 volts).
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it won't learn around the o2 tweak like it would learn to deal with a bigger set of injectors, different maf, etc.

The simple reason is because you have altered the o2's switch point, which is typically what the sensor is looking for.
As for the lean condition causing the car to overheat, 2.4 takes into account coolant temps and will tweak timing and fuel if needed.

I suspect however, that this is for his e30, which is Motronic, and as such probably only learns in the same temporary manor that lh 2.2 learns.
 
OttoB said:
I guess you'll get fault code 2-1-2 from LH, because closed loop doesn't understand why changes to afr doesn't affect 02-sensor signal. And because NB-lamda is only a switch, afr is controlled by its on/off-cycle, so LH will go mad.
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it will switch. he's not talking about replacing the o2 sensor with a straight linear voltage feed.

Evo owners are doing this with the NB sim output on various widebands to achieve exactly the same thing: closed loop enleaning.

So, now that we've cleared up the "yes you can do it and yes it will work", isaac have ya done it yet?
 
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