RoninTT

<center><img src="http://pictureposter.audiworld.com/40482/af_calc_vs_o2_voltage.1.jpg"></center><p>This is from O2 sensor 1, normalized by a factor of 100 (75 = .75 Volts). The A/F ratio data points were calculated from MAF,Injector timing,..etc. Has anyone seen an O2 do this before and could it be faulty or is, perhaps, my ECU's AD converter bad? O2 readings seem like they have a floating ground...or it has become an oscillator.

I will have to log O2 sensor 2 along side sensor 1 next time...maybe that will give me a better idea.

Thanks,

Dave

RockeTT City

In your plot, the yellow looks a right O2 sensor but too bumpy. Most O2 sensors generate a voltage between 0 and 1 V as shown in your wave proportional to how lean to rich is your mixture. This is a close loop control system that tells the computer lo limit the mixture in the fuel system until reach a mid point normally about 400mV and moves around this point (very similar to the way your AC works in home where the thermostat would be the O2 sensor and tells the AC unit maintain the house at 70F), but in your case looks too bumpy (from 0 to 800mV with a period of some seconds).

I don't have experience with the TT yet but in an old Toyota I use to have the veriations were a lot softer in a +/-150mV range around 400mV. When the car is cold start slowly from 0V while warms up until the 400mV (lock point) then start moving around there. The 02 sensor needs to be warm before generating voltage.

Normally when a O2 sensor goes bad, you get either 0 V because a short, or 400mV very steady that can be an open sensor (the computer default to the 400mV value) If I have to guess something with my limited experience, I would look to the fuel system or even some valve in the intake or turbo system.

Please fell free to correct me, Iam learning here!

Rick.

TTschwing

<center><img src="http://images.fotki.com/v1/photos/7/7305/130918/AFandthrottle-or.jpg"></center><p>You can determine A/F directly from the voltage output of the O2 sensor. I think I posted that last night.

RoninTT

I need to check the other sensor(2) though and compair the two...If the other is not going ape**** like this one, I'll know sensor 1 is probably faulty.

RockeTT City

Yes I agree, there should be some matching, at least talking about stability between the two sensors

TTschwing

O2 #1 does most of the work RE: ECU feedback. O2 #2 after the cat is kind of a watchdog to confirm all is well.

RoninTT

I would probably notice a signifigant change in power if the engine was running as lean as the O2 would indicate, but the engine pulls very hard and very smooth...after the boost kicks in . Also, the calculated A/F is pretty stable compaired to the O2 readings, if a little on the lean side.

john s

ECU doesn't really care about what the voltage value is, just whether it's lean or rich. It's a 2 state system going from rich to lean or lean to rich as quickly as 3 times a second, quicker than the sample rate of VAG-COM. So you're not really seeing all the variation. Pulse ramp is used to generate switchover and switchover delay is used to hold near stoichiometric. Check archives for graphs of sensor, control and injector signals.

RockeTT City

Ok, if the ECU doesn't care about the voltage, how it knows if gas is too lean or too rich?

RockeTT City

an oscilloscope, if you know somebody with one. The multimeter should be fine enough at least to see if there are such big variations in voltage.