I think you have made a mistake not putting any anti freeze in your cooling system,it is there to stop corrosion etc building up internally, whether your problems are due to this is difficult to say but if you can`t see any leaks then it must be using water within the engine itself, you will have to decide if the cyl.head has to be removed to check the gasket etc and in future I would use a 50/50 mix of a/freeze/water regardless of wherever you live, good luck.
cdc
January 2015
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