318 is not hot enough
180* Thermostats open at 180*.
So……. If your coolant is circulating fully you are:
At lest least at 180* and your gauge is wrong.
or
Your thermostat is opening early (or stuck open all the time).
Replacing your 180* with a 140* would do absolutely nothing or make things worse.
1. Verify the coolant temp with another measuring device (meat thermometer, another gauge).
If it is really 150*, replace the Stat with a new 180* unit.