You tryed adjusting it i take it? I disconnect the valve cable under the hood at the firewall, then I turn my knob to cold on the dash, then I set the heater valve to close. Then I hook back up the cable and lock the clip in place. Mine stays closed fine. You might have a bad valve or something, I see that from time to time. If you want to know for sure use hose pinch plier's and pinch off the heater hose after the valve. Then see if you get a cooler temp. reading. I do this on every car when checking the a/c performance.