2004 Mitsubishi Eclipse Heater

Suspect air pocket in the heater core. Remove the radiator cap, turn the heat temperature control to the hottest setting but keep the blower fan off. It helps to have the front of the car higher than the rear so park on an incline or jack the front of the car up. Run the motor for at least 15 minutes to reach operating temperature. Rev the motor to 2,500 rpm and watch the radiator coolant for air bubbles. It helps to squeeze the upper radiator hose while revving the motor to get the air out. When you think you have all the air pocket removed let the motor idle and check the heater temperature at idle. You can turn the blower fan on to the medium speed to check for output. If it feels better, shut the motor off and look into the radiator neck to observe the coolant. If the coolant pulsates up and down you still have a bit of air in the system. If the coolant stays level, put the cap back on and you should be OK. Check the reservoir level the next day before driving and set the level to the max mark. If you have the problem again in a couple weeks time chances are that you have a leaking head gasket. Mitsubishi head gasket problems are not common unless the motor has been overheated to an extreme.