The ECM won't go into "closed loop" until the coolant temp, AND the oxygen sensors are up and running. Normally, 185 or so on the coolant temp will allow closed loop operation, as long as the O2 sensors are jumpin. If not, it will/may never go into closed loop, which would effect the timing, and fuel injector pulse. Continuous open loop running would mean it is runnning rich, with little advance in the timing. Not exactly the best thing.
The prom can be upgraded to a reprogrammed one fairly easily, and this would allow for the lower coolant temps, basically letting the ECM obtain closed loop status at a lower temp.

[QUOTE=Adams' Apple]The ECM won't go into "closed loop" until the coolant temp, AND the oxygen sensors are up and running. Normally, 185 or so on the coolant temp will allow closed loop operation, as long as the O2 sensors are jumpin. If not, it will/may never go into closed loop, which would effect the timing, and fuel injector pulse. Continuous open loop running would mean it is runnning rich, with little advance in the timing. Not exactly the best thing.
The prom can be upgraded to a reprogrammed one fairly easily, and this would allow for the lower coolant temps, basically letting the ECM obtain closed loop status at a lower temp.[/QUOTE]

I'm looking at a dual electric fan set-up for my F-150.  This particular kit has one fan you set to come on at a lower temp than the second.  It also will kick on the high temp fan when the A/C is switched on.