I have a Mini Cooper S 2011.

If I start the engine and let it sit idle it slowly heats up and at around 200 degrees the overflow tank start to fill up from the top until it starts coming out of the cap.
I am not sure if this is a thermostat issue or something else.
I just replaced the coolant pipe from the thermostat to the water pump as well as the water pump.

Thanks for any possible input.

 

I just went through this. Replaced head gasket and that fixed the coolant coming out of cap issue.

 

I've also had this happen due to a bad cap on the coolant tank. I'd suggest starting there to see if that resolves it.

 

Agreed. Check the cap 1st. They are designed to hold some pressure hence the use caution when opening hot. A blown head gasket can cause the issue as well as it puts higher pressure into the cooling system.

 

Thanks I will replace the cap and go from there.