460 EFI open loop

I discovered that the 460 EFI engine in my 1992 class C motorhome is going to open loop at around 7" to 8" of absolute manifold vacuum (i.e. the O2
sensor indicates steady rich at that vacuum). I would expect the engine to go open loop closer to 4" or 5" of vacuum when I really have my foot hard on the gas. Is this normal for this engine, or is there something else I should be checking?
John Halbur
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