I've found that adding headers and not changing the tune will usually make the a/f about 3 points richer. (3/10ths of "1")
I'm not sure what a ranger PCM can/will/will not do with reduced back pressure. I do know that it will over time learn and adapt to it's environment.
So if you cleared the computer I'd *think* the PCM would in short order lean up the mix trying to get the a/f back to it's target.
btw, it does that not only with fuel & the idle air by-pass, but with timing too. Not WOT timing.. but part throttle timing.
He who is enslaved to the compass is free to sail the seas