Check the circulation in the system. With the engine warmed up & running at a fast idle, wrap your hand around the heater hoses in various places. The heat runs from the hose by the therm housing. If that is hot go to the return hose. when ever the hose is cooler on one side then the other it indicates a blockage at that point. The hoses should all be so hot you can not hold onto them for over 2-3 seconds.
George
