. Check radiator cap see if seal still good, only cost $10-20 for a new one.
. Check for signs of leakage from water pump.
. Pull radiator out, send it to a Radiator shop for removal of of tank and a good flush.
. When re-fill coolant, jack the car up a bit, runs heater at full blast, bleed coolant from the screw on the engine block.
. Check or test Thermal Stat.
. Still don't work, a leak down test will tell.