if there is no power loss cnsumption is well then there could b a possibility tht ur head part needs a job only(valve,guides,seats,facing nd a gasket)...
i wuz suffring from the same situation in my preowned santro nd rings had to b replaced too...it costed me exact 10,000rs including labour etc....
to get it confirmed.....when ur head part gets opnd....look it by urself nd ask the mec to tell u abt the problem,rings tbe change hon gy if there will be a play btween piston nd sleave.get it chekd by urself....hope it helps u.