The Z still runs like a dream I'm confusedHead gasket failing is a serious problem on any car you will get coolant temp engine warning lights ECU codes & generally it will run like crap because the plugs will be getting damp as the coolant mixes with the fuel as the gasket fails It may be a coolant system leak relating to the water pump (mine has been changed as the bearing gave up) which are a generic fault on the RCZ s plus coolant sensor/thermostat housings are another know suspect to die off