My son has a 1999 mitsubishi galant. The heat will only blow warm for a couple seconds,then goes cold or lukewarm. I replaced the thermostat and that didn't help at all. Would it be the heater core? I noticed one hose is hot and the other is barely warm on the heater core after the car has been running for a long time. Would it help to flush the heater core? How hard would that be? the heater core isn't leaking but I guess it still could be not working properly? Thanks for any help!!

 

It's probably plugged, I had one yesterday in a GM truck. Put a garden hose on one heater hose, not much happened, put it on the other and it blew out a big pile of orange goop. The truck has heat now.