Domenico,
I would check your MAP sensor on your OBD-1 system.
I take it this is not a California truck.
Your Map sensor alone should send it to open loop at WOT.
What is your MAP sensor reading in your scan when the scan says O/T?
Or do you ever get a O/T?
As you know the throttle (TPS) has three positions, C/T, P/T & O/T.