Recently I changed my thermostat because I noticed some sharp peaks in temp when I run the car. I put the new thermostat in (oem Mazda) and still goes up every now and then. And I don't leak any coolant, but when I turn the motor off I can hear the water boiling almost....some goes back in the overflow tank...and the rest just stays in the hoses...Any suggestions? Water pump maybe? I was also thinking how often do temp sensors go bad? Could that be the reason? Thanks

 

You could go through the list like i'm doing.

Radiator Cap, Thermostat, fresh coolant are the easiest to do that usually solve the problem. I don't know how the S5 radiator is done, but i'm also having seepage from the Coolant Level Sensor which may affect things.