After verifying the engine is running at normal operating temperature, the coolant flow through the system should be checked. The engine should be started, the heat should be turned to full heat, and the blower should be turned to high. If the system uses a coolant flow valve, then the temperature should be measured on both sides to see if there is any drop in temperature from one side to the other. It should be the same temperature on both sides of the coolant flow valve.
With the vehicle still configured the same way, measure the temperature of the heater hose or pipes at both heater core connections. The temperature drop across the heater core should be 10 degrees or less. If a temperature drop of more than 10 degrees is found, then the heater core is restricted and should be flushed or replaced.
