Volvo S70 Forum : “So, I'm thinking my thermostat is stuck open, or there is an engine temp sensor ...”

So, I'm thinking my thermostat is stuck open, or there is an engine temp sensor that isn't working.

Getting on freeway when cold in morning just slowly driving it and setting cruise warrants a 1 mark above cool with heater set to max.  With heater off it will heat up faster having no heater core flowing. Around the town going through gears and accelerating and sitting at lights it warms up completely normal.  It just heats up really, really slow when your not giving it hardly any gas.  Is this normal? Perhaps my gas mileage will suffer with incorrect operating temperature fluctuations.