Should the coolant overflow tank be at the full line when cold on a 2003 highlander?
Full to the level of the lower of the two lines that are molded into the overflow tank.
Why would you hear a gurgling sound when you step on the gas with the heater being on with a 2003 Honda Civic?
THATS WHAT THE OVERFLOW/EXPANSION PACK IS FOR. WHEN ANTIFREEZE/COOLANT BEGIN TO BOIL IT EXPANDS AND IT NEEDS A PLACE TO GO. THERE FOR IT GOES IN TO THE OVERFLOW/EXPANSION PACK. THIS PREVENTS DAMAGE TO THE ENGINE/RADIATOR AND OVERHEATING. THERE IS ALSO THE POSIBILITY OF A MALFUNCTIONING SENDING UNIT/GAUGE.