Engine not reaching operating temperature

If you want to test your thermostat, push the valve open a bit and put a string through the opening; release the valve so the string holds in place. Suspend the t'stat in water on a stove and bring the water temp up slowly while monitoring temp on a candy thermometer. When the t-stat starts to open, it will drop off the string. The t'stat running temp rating will be about 5-10 degrees F above the temp at which it drops off the string.

With an old T'stat, you should be replacing it anyway. Get a Stant brand (from NAPA if you have them up there); too many junk t'stat out there nowadays (As TMM notes above).

After the t'stat, the next most likely is that the temp sender is bad, assuming the guage checks OK. At 180F, it ought to read around 30 to 40 ohms from terminal to case.