shoebox

Running the engine will not purge all the water out. As long as you flush long enough where water can say go in the radiator and then out somewhere on the engine (like a disconnected heater hose) you will get pretty well purged. Keep your opti dry during all this.
If you remove the thermostat and put the housing back on, you can flush with the engine off, as the thermostat won't be there to block flow to/from the radiator.

94zLT1

thanks shoe..i guess the question im asking is, is can i drain the entire system (block and radiator) just through the radiator petcock if i take out the thermo and reattach the housing?

shoebox

It might take a while, but eventually you will be able to swap out all the fluid (using a hose and leaving the radiator drain open). However, you will not be able to drain it (block) completely without removing the plugs.