Originally posted by SleeperZ:
To fully flush the system, heater must be turned on to allow full flow to the heater core.
Why? There is flow through the core with the heater on or off. There are no gates or valves in the coolant lines or withing the heater core. Show me a schematic or picture that indicates otherwise. Go bust open a core from a junk yard if you want.
Originally posted by SleeperZ:
And yes, lower temp t-stat does effect warm up time.
Wrong. No matter what T-stat you use, they're all closed at engine start up. They do not open until they approach the operating range specified by the t-stat. Which means the engine is warmed up and at operating temp before the t-stat does ANYTHING. Check the operational temperature range for the Zetec ~ 145 to ???.
Originally posted by SleeperZ:
Also, a too low of a temp t-stat won't allow enough time for heat to transfer from the coolant to the radiator.
A lower temp t-stat does not magically speed up coolant flow through the radiator. That is controlled by the water pump & the size of the openings in the t-stat, radiator specs, coolant hose & fitting sizes. Having seen the difference in the 160 & 185 t-stats first hand, there will be no significant difference in coolant flow, if any.
The only valid point you might have is the engine is/may not be able to maintain the 160 temp when there is low air flow over the radiator. But then guess what??? The fans kick on at their pre-programmed temp and all is right with the world. Then when you get moving, the fans shut down & the engine temp drops back to around 160 again. Cool, huh?? 
Must be that jumbly-wumbly thing happening again.
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