Conditions to enter closed loop when @ idle

I have 93 C1500 Suburban 5.7L K L05 with 16168625 pcm and BDJZ code. I'm trying to chase down some intermittent problems. I used Tunerpro to log.
I started @ idle and let it warm up to Engine Temp reading over 160 - IAC around 25, spark advance around 20. It never entered closed loop. As soon as I gave it a little throttle it went closed loop and then dropped out again @ zero throttle.
After driving and Engine Temp read 189, it was in closed loop @ idle - spark advance 15, IAC @ 0
I had read somewhere, I thought, that over 160 it would go closed loop, but I assume there are multiple criteria.
I've googled and not been able to find a summary of conditions necessary to enter closed loop - hoping someone with experience can point me in right direction.
Thanks
Ed

Possible bad or weak o2s

if it don't have a 3 wire o2 sensor install one ,my 4.3 would go open loop at 30mph in 5th 3 wire sensor fixed it.if it has a 3 wire check for exhaust leak if engine's not been modified. or as stroked said

I see there are 2 "loop" modes monitored in TP, one is "Loop Status" and the other is "Idle loop mode". Are you sure you are looking at the right one?

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Originally Posted by damanx

I see there are 2 "loop" modes monitored in TP, one is "Loop Status" and the other is "Idle loop mode". Are you sure you are looking at the right one?

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There is a "closed loop" flag in TP. That's what I'm looking at. It runs closed loop when driving and at idle when hotter ~ 180ish.
I think O2 sensors are OK, seem to vary while running.
I was just curious as to the actual criteria used in ECM -haven't been able to find specific detail when googling around