Anyone Know about the different fuel modes

I've got a 2000 Century that has no acceleration and it will not ilde when cold. With use of a scan tool I noticed that my problems only occur when in the Open, Open1, or Open2 fuel modes, never in Closed mode. My understanding is Closed mode is normal driving mode after warm up, Open is at first start up and until O2 sensor heats up enough to give correct signals, and Open1 & Open2 are for heavy acceleration and decceleration.

I noticed under load that my O2 Sensor readings drop to less than 100mV and tend to hold steady just prior to the fuel mode swithing from Closed to Open1 or 2, then O2 sensors drop to zero mv and vehicle will only idle. Most times stopping, turning of the engine for 10secs and restarting resets the fuel mode and all is good until the next steep hill. Problem occurs less frequently during the colder weather, but still occurs. Gas mileage is down from a very steady 30mpg hwy to 27 - 28 mpg. 3.1 Engine has 200K miles on it.

Any ideas? Thanks,

First thing I would look at is fuel pressure when this occurs, particularly under load. Any codes? Also check tps and maf activity, but FP first.

First thing I would look at is fuel pressure when this occurs, particularly under load. Any codes? Also check tps and maf activity, but FP first.
No codes are set unless I continue to try and drive the vehicle in it's "no go" mode, at some point I get an O2 sensor low voltage code. I'll check fuel pressure next, any idea what it should be?
Thanks

41-47 psi.