sir,

if the cap hit something,it will..

i think baka di lang ganun ka-tight paghigpit,umikot due to engine vibration,tapos un..is the engine bay oily(full of engine oil)?most mechanics i've talked say this is normal..due to engine activity inside...better buy an oil cap,baka mapasukan ng water(moistures in the air) magkaprob ka pa lalo..madami naman sa surplusan and merong brand new..

hope this help...