Conditions to enter closed loop when @ idle

[Stroked 388]

Possible bad or weak o2s

[ony]

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