A brief rundown: I know that my thermostat needs replacing, but it hasn't been an issue yet. It was going to be replaced next week, since my mechanic said it wasn't urgent. Though the other day, I notice that the car is running hot. Almost in the red. And the heater vents are blowing cold. So I figure that I've got no coolant running through the engine. Which I don't. Actually, it's all over the engine well and the pavement. Not good.

Now all my hoses and clamps are OK. But at the top passenger side corner of the radiator there is an open hole. And it looks like my coolant is spewing out of that.

I've looked for pics and diagrams online and asked other Volvo owners with zero luck. I have no idea what that hole's purpose is. Is it a release valve? Should it be plugged? Or has my thermostat finally frozen and caused a greater problem?

Thanks in advance.