There is no flow from stone cold to climbing Everest. I did this both with the cap off, and then with the hose off just in case I missed a weak flow. As far as I can see, no scale on the fins (the few that are visible), and the coolant ran clean when I emptied the radiator prior to removal. I had good flow through the radiator when connected to a garden hose (both through the top tube and bottom) so, I would think that I would get hot water at the bottom hose when connected to the engine. But again, no flow. This was a "ran great yesterday, failed today" type of situation, not a growing problem which also has me baffled. One thing I failed to mention (just remembered), is the rad cap had exploded, the spring and plastic part that held the spring on were inside the radiator; they came out when I flushed the radiator. Not sure about any rubber bits from the seal though.