my thermostat was not opening, so i changed it. i found white residue in my resivoir and on the top of my rad and cap. Is something wrong? and I also have a problem, the car won't hold heat, and when i drive the needle goes all the way to C. is the thermostat not set properly. I did the best i could, but it kept dropping out of place. Is there i way to solve that?

thanks in advance, for all ur help

 

bad heater core?