Hi Matthew
The control under the dash is IN for ON & OUT for OFF. I find that the water is still getting hot because only one side of the heater rad has a control valve. Sometimes these stick so it would be a good idea to set it to cold under the dash and then try and move the valve lever at the rear of the cylinder head. You could try slackening the control wire at the valve and try to close it a tad more. I hope this helps
David
