its better to run with one....if its running to cool you run almost the same risks as running hot....

example...go spray a light bulb with a squirtgun...it will break....gaskets do the same thing and heads warp...

buut you might just wanna check your temp sender...ive had three hondas that they went bad on and gave false readings...

or maybe your thermostat is backwards, or just bad. or maybe your fan switch is falty.

did you bleed the system when after you put it in? heater core included?