How do I test if my Head Gasket is blown? My car has been over heating once in a while. I have plenty of coolant, new water pump, new thermostat, and rebuild radiator. I notice when my car over heats, there's a bubble coming out of my reservior. My car is not smoking any sort of color.. white or blue. any feedback is appreciated.

 

if you're not loosing any coolant, and there's no smoke, your head gasket's fine.

 

The coolant overflows at the reservoir.

 

It sounds an awful lot like a bad thermostat. I know you said it was new but you may want to try another one. Also are you fans coming on? If not that could be the culprit.