O2 voltage help

[jimh]

I installed a GM MAFT yesterday in my 90 TSi and when I hooked up my logger to start getting my fuel trims tuned I noticed that my O2 reading is coming in at 4.98V (after engine warm-up). Each of my fuel trims is at 100% because I reset my battery and at idle my O2 reading remains at 4.98V. While cruising my O2 varied from 4.98V to around 2.50V but the change was not the normal, quick oscillations. The fluctuation of the O2 reading was very spread out (large wavelength) and it did not appear to vary at all with TPS. What would cause my O2 to show such high voltage when it should be between 0-1V? BTW, I swapped in another O2 sensor with no change in the reading.

FYI: stock fuel pump/injectors, 2.5" exhaust

 

[danl]

That is extremly weird. Has the O2 sensor wire ever been tapped for an O2 voltage gauge? Is it possible that the signal wire is shorted or comprimised in any way? Is it possible that the wire connection is broken and the signal wire is floating?

If you send much more than 5vdc to the signal wire you could damage that input to the ECU or possibly the whole ecu.

 

[WhiteTsiAwd]

Well either you have it hooked up wrong, which is very easy to do, or its bad. Thats really about the only 2 options. Well or a short but I doubt its that, well hopefully its not.

 

[jimh]

There was an O2 sensor gauge hooked up to the ecu for a couple of years and it worked properly. I have had it disconnected for several months and the wires are capped off seperately though still tapped to the ecu. The sensor was working properly the last time I logged my car which was about two months ago. Since then, the only change I made was when I installed the MAFT this weekend. I guess that the place to start looking is at the wiring for the O2 gauge to determine if there is a short somewhere, right?

 

[WhiteTsiAwd]

Ya hook up the gauge or preferably use a logger to see if you are getting the correct voltage and to see that it is cycling properly. But if you said that you used a logger and you were getting readings of 4.98 or 2.50, then you either have it hooked up wrong or it is bad. Trust me, I struggled with this problem for 3 months before I finally got it all right.

There should be TWO white, ONE black and ONE grey (on the O2 sensor side) and ONE red, ONE white and TWO black on the car side.

Ok, you hook them up like these:

O2 sensor side.................Car side.
Grey--->White.
Black--->Black.
White--->Black
White--->Red

Thats how it all hooks up so if you have it all hooked up right then your O2 is dead.
Good luck.

 

[jimh]

Thanks for your help and your suggestions. I detached the wiring for the O2 gauge from the ecu harness and now the O2 reads normally again! So there must have been some compromised wiring connection but all is well now. Thanks again for the help. Time to tune!

 

[WhiteTsiAwd]

Well glad you got it working. Good luck with tuning.