i am 100% sure that your head(engine head) is not faced perfecdtly and the gaskit is causing the water to overheat and evaporate... so please check this as soon as possible....
i am saying it again its a common issue with santro and other efi cars, when after a engin overhull the head is not properly faced(grinded to smoothness) by the kharadiya causing the gaskit to be on a uneven surafce and thus causing water to evapurate////
so bro check this....
with all the best wishes,
ahmad