Is the O2 supposed to do this?

[mecevans]

I was messing with the scope today trying to pinpoint my missfire when the engine is hot and idling. Knock sensors, CKP, pulse width, IAT, ECT,TPS, and air flow signals are all operating normal. The O2 sensor however drops below .1v at idle. Now i know its not supposed to do that on obd cars, but what about the supra? The tsrm says to only test O2 at 2500rpm. As you can see it stays rich at wot, and goes lean at fuel cut like it should. It also behaves correctly with load applied. And as far as my misfire at idle im getting no detonation, or pulse width fluctuations so its safe to say its not a fuel control problem and most likely spark?

[video=youtube;aAh3Y_zdhmA]https://www.youtube.com/watch?v=aAh3Y_zdhmA&feature=youtube_gdata_player[/video]

 

[CyFi6]

A misfire will cause oxygen to be dumped directly into the exhaust along with the unburnt fuel, but since the o2 sensor can only read oxygen, that is likely why you are seeing the o2 voltage go down to 0. BTW the Supra is does have OBD . When you rev it up it looks like you have at least about 8 switches up and down within a 10 sec time period, and when you snap the throttle it reads rich, so I would say your o2 is working fine and simply reporting what it sees.

 

[mecevans]

The misfire im seeing and hearing is only a partial one. Its also not consecutively, maybe once every 3-5 seconds. Im going to test the heater circuit maybe its cooling off at idle. Ill also use some propane to try to richen it and see if it responds.

 

[jetjock]

You should still have cross counting at idle. It'll be slow though. For misfire the best thing to do is secondary analysis. It'll tell you which cylinder(s) is at fault.

 

[Nick M]

For future refrence, the feedback circuit, in general, is the last place to look for a misfire. I think that is even in one of the TM's, the one that shows how to use the Vetronix.