Just a quickie question. During a datalog, I had the ECM go O/L without any seeming reason to. This was only at idle, with Coolant Temps well within C/L range. Once I tapped accel, it quickly went back into C/L. This happened only once in a 70mi logging session w outside temps at 47-50F. Z

 

Heated o2 ?

 

Yes.

 

If the narrow band O2 stays within an upper and lower limit for too long the ECM can think it has stopped responding and drop into open loop. To an extent the ECM expects this to be possible if the original sensor was non-heated and ambient temps are low enough so it doesn't throw a code.

GD

 

I recall this happening w my 84 Xfire which had a non-heated O2 sensor. I later updated it with the installation of EBL.
The ambient temps today were in the high 40’s and the LT-5 has 2 3wire sensors. As I said, it ran fine for 70+ miles. However I did note at initial startup that it took longer than usual for it to go into C/L. Should have gone at 60C coolant but finally flipped to Closed when coolant was at ~ 75C.
BTW, this is with a motor that just had a “new” short block installed. First time datalogging it.

 

Could be an O2 sensor getting lazy or fouled or a poorly working heating element. Does the LT-5 even have the ability to detect heater circuit malfunctions? You might want to continuity check the heating elements.

GD

 

No heater circuit check. I did take a look at the log at the point where it was sitting in O/L. This happened only while sitting at idle. The xX-counts on the left bank went to 0 and as u said, the ECM went to O/L shortly after that. In comparing L/R O2s, they appeared to be tracking closely w each other for the rest of the log. Taking a closer look at the log, I see this was happening more than once, and that even when in C/L, the left O2 sensor Xcounts would drop back to 0 after reaching 64 counts while the right side would continue on to 256. Perhaps new O2s are in order.

 

Heating elements are the most common failure item on O2 sensors.

Since it's occuring at idle when the sensor cools down I predict you will find the heating element for that bank's O2 is open circuit.

GD

 

Got the AC Delco O2s yesterday.

 

AC O2s are installed. Just waiting for decent enough weather to test, altho I did check voltage with ignition ON but no start.

 

Reviving this thread to update. When I first replaced the O2, I only did the right side since it appeared to be the one that was dropping pretty low into 2 digit mv range. My thinking was that this was causing the ECM to drop into O/L. For the most part that seemed to work. But in recent logging sessions, I began once agin to see periodic log segments showing O/L even during cruise portions of the log. And almost always going O/L at idle disabling Learn Control. Looking st the log more closely, I realized it was the Left side, not the right, presenting a lack of Xcounts. Always on the lean side.
Installed 2 new AC O2s yesterday, and now solid C/L throughout the log session including idle. Looks like left side was dying off over time, but no idea as to why.

 

Did you check the heating element wires for continuity?

GD

 

In the O2 sensor, itself? Not sure how to do that, but I did check for continuity in the engine harness.

 

Right - in the O2 itself. You can just check the wires with an Ohm meter. There should be some resistance but if it's burnt out it will be infinity or very high.

GD

 

Which wires for the heating element. I have 2 white wires in A and C, black in the B.