> <<Your vehicle has some problems if the temp rises when you just open the
> rear valve. >>
> 
> I think the problem is that you are not considering the consequences of the
> mass flow through two elements of vastly different rates of heat transfer under
> the different conditions.

Or, to put it another way, if the rear valve is open without the rear
fan on, it acts as a non-cooling shunt taking flow away from the
coolant circuit through the radiator. The jackass PO of my vehicle had
a rear core leak and replaced it with a short loop of hose. Even after
a new radiator it always seemed like I was running hottoer than
everyone else (the other coolant shunt through the AT cooler didn't
help either). Once I finally learned how the coolant circuit worked
and replaced those metal tees in the heater circuit with straights and
eliminated the rear heater entirely, the temp needle actually ran
visibly lower.