I installed a 3 wire two seasons ago. Was working on the car today and started it up for the first time since it was put away for the winter. It was running fine, then when it went closed loop the O2 kept cutting out. I could see it on the wide band as well because I have an AEM gauge in the pillar that monitors the wide band for tuning. It will sit at 14.7 then it jumps to 16, then off the scale as I just see dashes on the gauge. I can hear it and feel it, car sounds like it's bogging, then it comes back online and runs fine. Few seconds later it cuts out again. I did spill some oil down the header on the O2 side and it's burning off, not sure how that would cause it.

 

Have you put a scan tool on it to see what it is doing?

O2 sensors have a breath hole in their body so outside air can can enter the O2 sensor or they breathe through the wires. Maybe the oil ran down the O2 sensor's wires or onto the body of the O2 sensor and is clogging up the breath hole.

I recommend 4 wire heated O2 sensors for headers because on the actual O2 sensor side they don't ground through the sensor's body. They have a dedicated ground wire to go along with the signal wire.

Double check to make sure the O2 sensor is still tight in the bung on the header and make sure the headers bolts are still tight too.

Heated O2 Sensor - Third Generation F-Body Message Boards

 

I had no idea about the breather. Thanks for the info. I'm pretty damn sure the oil went down there by the O2. Spilt the bloody stuff when I was pouring it and it ran down the back of the block and off the valve cover between the #5 and 7 cylinders.

 

I'm waiting for my OBD cable to arrive so I can scan, in the meantime I tried a new 3 wire O2 sensor as mine was coated in oil. Same issue. Car starts up and it's lean off the hop. I hear it go closed loop then the AEM reads 17 and goes off the scale. At the same time the idle drops down to barely running then comes back. It oscillates like that on about an 8 second interval. If I crack the throttle it will go rich to around 11 then back lean off the scale. I did a valve adjustment so I had the plenum and runner off on the one side, MAF disconnected and plumbing etc. Pretty damn sure I put it all back together properly. Also replaced the alternator with a 140AMP one so I'm not sure if any of that is the issue. My volts are fine on the gauge and everything is working in the car. Seems kind of strange to all of a sudden have this issue.

 

*UPDATE*

Got it sorted. It was the 6,8 big runner tube gasket at the base. The edelbrock runner isn't quite cut perfectly so the big tube gasket I bought was sitting to high and there was a small gap across the top and it was sucking in unmettered air, screwing up the 02. I got some stock felpro gasket material and traced out my own gasket. O2 is back at 14.7 closed loop. FYI, the old propane trick worked yet again.

 

Glad you found your problem.