Some people have better results when the valve is installed on the supply hose going to heater core rather than the return hose going back to the engine. Try swapping the hoses and see if that helps. I found these valves to be pretty useless as you have discovered. I replaced my valve with an electric unit mounted by the ac compressor. It has 4 hose connections so even when the valve is closed, water still flows via through the engine heater hoses but not to the heater core. The 4 port valve does not stop flow it just diverts it. It works well.
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