CHECK THESE LAST:

11. LEAKING HEATER CORE
Have heater core professionally replaced.

12. FAULTY CYLINDER HEAD GASKET
Have cylinder head gasket professionally replaced.

13. FAULTY HEATER WATER CONTROL VALVE
Have water heater control valve professionally serviced.

14. DAMAGED COOLANT RECOVERY RESERVOIR
Have coolant recovery reservoir professionally replaced.

15. CRACKED CYLINDER HEAD, INTAKE MANIFOLD OR ENGINE BLOCK
Have cylinder head, intake manifold or engine block professionally replaced.