318 is not hot enough

Thermostat ratings are measured 'fully open', so in other words ...the rating is telling you thats when the thermostat is fully open, otherwise they 'start opening' [180* for instance] at 150-155*


It is possible to have too much radiator capacity, but that would show at cruise/freeway not idle.

Running too cold is not good just as running too hot can be worse, either is still not good.

What water pump are you running, and 1 more time...is the stat opening or not?

Take it for a spin then check for opening...