So far ive only replaced the thermostat (which was stuck open). The cooling fan comes on when i turn he AC on. But if i dont have the AC on, the fan wont come on and the coolant will get so hot it starts to boil.

What else can i replace, or check?

 

Replace the engine coolant temperature sensor (ECTS). It sends the signal to the computer to turn on the cooling fan. Good idea to replace the connector for the sensor at the same time (they go bad over time from the heat). Total parts cost will be around $40USD. Sensor is located on the driver's side of the engine, below the thermostat housing (gooseneck). Use Teflon tape on the threads of the new sensor, and make sure it is an OEM-type with a metal (brass) tip. There are some "cheapies" out there with a resin-tip, but they don't last ...