355 Is my engine running too cool?

Totally normal.

Most cars these days (at least the ones that have actual temp gauges) rise to the middle and stay there by design. The F355 gauge actually functions like a gauge and not like an idiot light.

if your thermostat was bad it will take a very long time to get the gauge off full cold.

 

Mine used to do that. Changed the thermostat and temps became steadier. Its a cheap and easy job to do.

 

Compare temperature readings using a scan tool on the OBD connector. The instrument cluster gauge receives its signal from the temperature sender, a thermistor type. The readings at the OBD will be from the ECU, a signal from the temperature sensor. The sender and sensor are located close to each other, forward of the oil filter near the heat exchanger, under the coolant recovery tank.
Normal operating temperature is 190 deg.F. Also check the tightness of the nuts at the flex circuit board behind the gauge. I was experiencing the same issue. I found mine to be less than finger tight.

 

Mine has operated the way you describe since new. I like engines to run a little cool.

 

348's and even 360's do that. The question is when your not moving or stuck in traffic does it go back up to the 190 range.
the newer thermostats are a little hotter and open a tad later. I struck a new 355 thermostat from Ricambi in my 348 and it went up 5-10 degrees.
Cars running too cool is not a good thing cars running too hot is not good. But if it is cool outside they tend to run a bit cool, as mentioned the new 355 thermostat I put in raised it 5-10 degrees across the board or so when running at 70 when its cooler outside. It also idles just above 190 when stuck in traffic. Prior it would never break 190 when stuck in traffic either.

348 manual lists 178 at the lower end of the operating temp if I recall and it was definitely there or slightly lower before the new thermostat. Check the 355 manual for operating range to be sure. good luck!

 

That is about where mine sits under the same conditions (cruising on mild days). It will creep up as I sit at a light, and then the fan takes over and cools it down.

 

As already stated, you need to use a scanner and not rely upon the dash gauge. While cruising, my scanner engine temp reading was 165'; much too low. The coolant temp sensor sends this low temp signal to the ECU, which then adds fuel to richen the mixture.

The key to my remedy was a new 82'C thermostat. Fuel trims are spot on spec, and after warm up, my low temp at any time is 183'.

I always thought the same thing, but you do not want your car to run cool.