mine would only mist up for 30 seconds before clearing, when the car was up to temp and the heater was then switched on.
i imagine all the steam was building up in the heater box before releasing it by sliding the heater control over (thus opening the box allowing the steam to escape)
in other words you can run it all you like once its up to temp but it will only steam up for the first few seconds upon opening the heater box (with a hot matrix)
oh and k-seal is a better version of bars leaks
