I have a ford expedition that has no heat coming out the front or back heater coils. THey have been flush and there wasnt any blockage and they had good flow. I have replaced the thermostat(which wasnt broke). The supply hose to the heater core hardly gets warm. I disconected the supply hose and ran the engine. There isnt any fluid flowing to the supply side (just a little bit when the antifreeze gets warm and expands). I flushed the supply hose and water flows out the return hose so i dont think there is a blockage. Also with the engine off iI can squeze the return hose from enigine to radiator and make water pop out the heater suply hose. Can this be a symptom of a weak water pump? The truck doesnt overheat. From extensive reading I dont think there is a control valve. What other investigations can I do to try and figure out what the problem is. Thanks for any help.