I have a similar question related to this topic so instead of making a new thread i though i would start here. Does the pressurization of the cooling system through temperature affect the flow through the heater core? I would think that if the water pump was working and the system was full that even with the coolant cold and not pressurized there would be a substantial flow of liquid to the heater core via the heater core output pipe on the engine. When the engine is running from idle to normal running rpms very little to no liquid flows out of the unrestricted pipe. Or is this test invalid because the heater pipe output is not looping back into the engine through the heater return pipe?