First of all, that's very irresponsible to be driving around if your temp guage is fucked up... get it fixed. The temp guage can be the difference in being stranded on the road or being able to catch a potential problem in time.chirpin_accord said:
Have someone check the Thermostat. If that's busted, no matter how much fluids you have, you will over heat if the thermostat is busted in the closed position.
As for the "melted plastic smell".. I have no idea.